Ultra-high-pressure metamorphic rocks in the Western Alps

Compagnoni, Roberto; Hirajima, Tsuyoshi; Chopin, Christian

doi:10.1017/cbo9780511573088.008

Cited by 84 publications

(89 citation statements)

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“…Although the structure of each orogen has its own peculiarities and stands as a case apart, collision orogens typically include an axial belt containing the remnants of stretched continentalmargin lithosphere, which have undergone highpressure metamorphism before being exhumed and re-equilibrated at greenschist-facies, amphibolitefacies or even granulite-facies conditions (Chopin 1984;Compagnoni et al 1995;Hacker et al 2006). Such an axial backbone of neometamorphic rocks, deformed at subcrustal depths, represents a fossil continental-subduction zone (Garzanti et al 2010).…”

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confidence: 99%

Metamorphic grade of source rocks revealed by chemical fingerprints of detrital amphibole and garnet

Andó

Morton

Garzanti

2013

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Amphibole and garnet are among the most widespread heavy minerals in orogenic sediments. Their chemical composition and optical properties vary markedly and systematically with temperature and pressure conditions during growth, and thus provide important information on the metamorphic evolution of source areas that is crucial in palaeotectonic and palaeogeodynamic reconstructions. This study investigates the chemical composition of detrital amphiboles and garnets derived from parent rocks of progressively increasing metamorphic grade through a well-studied composite section across the Central and Southern Alps, including the granulitefacies core of the Late Palaeozoic orogen exposed in the Ivrea-Verbano Zone and the amphibolite-facies core of the Cenozoic orogen exposed in the Lepontine Dome. We specifically focus on metamorphic grade because it represents the best proxy for tectono-stratigraphic crustal level, and hence degree of unroofing of source areas. In river sands collected between metamorphic isograds corresponding to crystallization temperatures ranging from c. 500 8C to c. 850 8C, TiO 2 gradually increases in detrital amphibole while its colour progressively changes from blue-green in the lower amphibolite-facies where actinolite, hornblende and tschermakite are most abundant, to brown in the granulite facies where pargasite is dominant. Detrital garnets display moderate gradual changes across the amphibolite-facies Lepontine Dome, where low-Mg 'type B' garnets predominate. Almandine-spessartine is spatially associated with abundance of pegmatites while entering the zone of anatexis (Southern Steep Belt), where grossular or grossular-andradite-spessartine are occasionally found. A sharp change occurs while reaching granulite-facies in the Ivrea -Verbano Zone, where high-Mn garnets disappear and 'type A' almandine-pyrope (from 'stronalite' metasediments) and 'type C' almandine-pyropegrossular (from metagabbros of the Mafic Complex) predominate. Also redefined in this article are a series of numerical indices based on amphibole colour and relative abundances of diverse key minerals (chloritoid, staurolite, andalusite, kyanite, fibrolitic and prismatic sillimanite), useful to accurately assess the average metamorphic grade of meta-igneous and metasedimentary source rocks.Supplementary material: Chemical composition of detrital amphiboles and garnets and full information on the location and mineralogical composition of studied samples is available at

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confidence: 99%

Metamorphic grade of source rocks revealed by chemical fingerprints of detrital amphibole and garnet

Andó

Morton

Garzanti

2013

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“…An upper normal fault and a lower reverse fault bound the thin-aspect-ratio slab. Such shear senses seem required by structural relations, for example, in the Dora Maira Massif (13)(14)(15), and in the Dabie Shan (16). Yet another exhumation scenario involves the antithetic faulting characteristic of some compressional orogens, in which double vergence is produced during end stages of the collision and ascent of sialic crust (17).…”

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confidence: 99%

Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust

Ernst

Maruyama

Wallis

1997

Proc. Natl. Acad. Sci. U.S.A.

231

120

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Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves underf low of a salient (250 ؎ 150 km wide, 90-125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancydriven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over Ϸ20 million years, rapid (Ϸ5 mm͞year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes ref lects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chief ly of transformed, yet relatively low-density continental crust compared with displaced mantle material-otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to f luids, and nearly anhydrous. In contrast, H 2 O-rich quartzofeldspathic, gneissose͞ schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds.

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“…It is interesting to note that since the BIU discovery (Chopin, 1984) the inferred peak T did not change, whereas the peak P progressively increased from about 3.3 GPa within the coesite stability field (e.g., Compagnoni et al, 1995) to about 4.3 GPa within the diamond stability field. Even though microdiamonds have never been found, the P increase of about 1 GPa has been inferred from both experimental petrology (Hermann, 2003) on representative BIU rocks and from a number of different geobarometers and thermodynamic modelling (e.g., Castelli et al, 2007;Groppo et al, 2007a).…”

Section: Suggested Petrologic Criteria For Recognizing the Uhp Biu Frmentioning

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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Ultra-high-pressure metamorphic rocks in the Western Alps

Cited by 84 publications

References 0 publications

Metamorphic grade of source rocks revealed by chemical fingerprints of detrital amphibole and garnet

Metamorphic grade of source rocks revealed by chemical fingerprints of detrital amphibole and garnet

Buoyancy-driven, rapid exhumation of ultrahigh-pressure metamorphosed continental crust

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

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