Garnet–chloritoid equilibria in eclogitic pelitic rocks from the Sesia zone (Western Alps): their bearing on phase relations in high pressure metapelites

Vuichard, Jean-Paul; Ballèvre, Michel

doi:10.1111/j.1525-1314.1988.tb00413.x

Cited by 58 publications

(24 citation statements)

References 49 publications

(76 reference statements)

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“…Estimates for T max value of 500-600°C for Alpine re-equilibration (Table 2) are in agreement with abundant literature estimates on metapelites, marbles, metagranitoids, and metabasics of SLZ (Compagnoni 1977;Lardeaux et al 1982;Hy 1984;Vuichard and Ballévre 1988;Castelli 1991;Pognante 1991;Zucali et al 2002;Rebay and Messiga 2007).…”

Section: P-t Estimatessupporting

confidence: 78%

Interplay between deformation and metamorphism during eclogitization of amphibolites in the Sesia–Lanzo Zone of the Western Alps

Gosso

Messiga

Rebay

et al. 2010

International Geology Review

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Interactions of fabric evolution and chemical parameters driving reaction progress during the amphibolite-to-eclogite transition were investigated in eclogitized amphibolites of the Western Alps. In the Sesia-Lanzo Zone (SLZ), mafic rocks ranging from eclogitized hornblendites to true eclogites occur as layers and boudins within micaschists and are characterized by different modal amounts of amphibole, omphacite, zoisite, garnet, and phengite, constituting the Alpine HP assemblage. Across narrow zones, this array of lithologies displays gradients in planar fabrics characterized by coronitic, S-tectonitic, and mylonitic textures with different extents of eclogitization. Eclogitic parageneses are controlled not only by the bulk rock composition of the protoliths but also by the degree of fabric evolution. This is the case for omphacite occurrence, which is constrained by plagioclase composition and modal amount (NK parameter) in the protoliths, whereas the increase in modal omphacite and the concomitant decrease in modal amphibole in rocks with high NK are controlled by the strain rate. Protoliths with a low NK content develop the amphibole + garnet + epidote assemblage in eclogitized hornblendites. In protoliths with higher NK values, the co-existence of amphibole with garnet, omphacite, and epidote occurs only for low-to-medium strain textures (e.g. coronites and S-tectonites), whereas an amphibole-free assemblage defines a mylonitic foliation; in this case, amphibole relics are present exclusively as armoured inclusions in garnets and omphacite porphyroclasts. Thus, amphibole persists in the eclogitic assemblage at pressures exceeding the experimentally determined amphibole stability field. Values of confining pressure under which Sesia-Lanzo mafic rocks re-equilibrated during Alpine subduction were estimated, through equilibrium assemblage modelling, at 2.2-2.7 GPa. The amphibole-bearing eclogites of the SLZ show that large volumes of amphibole-bearing rocks can be exhumed from a depth exceeding 75 km without dehydration reactions running to completion. Petrological estimates in orogenic zones may help constrain geological and tectonic conclusions when selection of laboratory samples is assisted even by simple microstructural evaluation of planar fabric development.

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Section: P-t Estimatessupporting

confidence: 78%

Interplay between deformation and metamorphism during eclogitization of amphibolites in the Sesia–Lanzo Zone of the Western Alps

Gosso

Messiga

Rebay

et al. 2010

International Geology Review

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“…Therefore, a part of the chlorite that overgrew primary minerals within the regolith can be related to the alteration of the HP minerals of the rocks of the Sesia-Lanzo Zone during its cooling history (e.g., Pognante 1991). Figure 4 shows that chlorite of ripidolite composition, which occurs as an alteration product of primary minerals within the regolith, corresponds to the chlorite composition described by Vuichard and Ballèvre (1988) for the retrograde metamorphism in the eclogitic pelitic rocks of the Sesia-Lanzo Zone.…”

Section: Discussionmentioning

confidence: 97%

The composition and evolution of an Oligocene regolith on top of the Sesia–Lanzo Zone (Western Alps)

Kapferer

Mercolli

Berger

2011

Int J Earth Sci (Geol Rundsch)

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An Oligocene paleosurface (regolith) lies on top of the high-pressure metamorphic rocks of the SesiaLanzo Zone near Biella, NW Italy. Only the saprock, the lowermost part in a regolith profile, is preserved. No evidence for any paleosoil can be observed. Field observations indicate that the regolith developed through in situ mechanical fracturing of the rocks of the Sesia-Lanzo Zone in a continental environment. Density estimations of the regolith and the underlying rocks of the Sesia-Lanzo Zone confirm the field observations and imply that a relatively small amount of alteration minerals was formed. The main detected alteration phases are chlorite, various carbonates, quartz, clay minerals, Fe-oxides, and Fe-hydroxides. Chlorite differing in chemistry and crystallographic ordering demonstrates different stages of alteration. Oxygen and carbon isotopic composition of carbonates suggests temperatures higher than surface conditions. Illite and chlorite thermometry indicates temperatures related to the anchizone (*250-300°C). These data are considered as a robust indication of the re-burial of the regolith together with its substrate and its volcanic cover. The burial is closely related to the tilting of the preserved stratigraphic sequence formed by the rocks of the Sesia-Lanzo Zone, the regolith, and the rocks of the Biella Volcanic Suite (Lanza, in Schweiz Miner Petrogr MItt 57: 281-290, 1977; Lanza, in Geologishe Rundschau 68: 83-92, 1979). Furthermore, the burial is consistent with this sequence of subaerial rocks being very close to the intrusion depth of the Valle del Cervo Pluton at the time of its emplacement (4-7 km;

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“…It is difficult to make a choice on which is the most reliable temperature for the peak metamorphic conditions; however, the values of 530°-560 °C obtained for all lithologies from the garnet-phengite thermometer [32] at 16 kbar matches the best T (and P) estimates given for peak conditions of the Eclogitic Micaschists Complex of the Sesia Zone (T = 500-600 °C, P ≥ 15-17 kbar [25]; T = 490-570 °C at 15 kbar [47]; T = 550-600 °C, P ≥ 15-18 kbar [48]) and of the Mt. Emilius klippe [35,36].…”

Section: Alpine History 43 the Alpine Eclogite-facies Eventmentioning

confidence: 77%

Geology and petrology of the Austroalpine Châtillon slice, Aosta valley, western Alps

2004

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Slices of continental crust pertinent to the lower Austroalpine domain of the western Alps, crop out within the ophiolitic Piemonte Zone. Among them, the Châtillon slice was studied in detail. The slice consists of orthogneiss with subordinate metabasics and very minor paraschist. The garnet-phengite-epidote-albite orthogneiss is characterised by polyphase garnet porphyroclasts. Metabasics consist of prasinite lenses and eclogite relics. Phengite-clinozoisite eclogite is characterised by small garnet idioblasts with prograde zoning; jadeite content in omphacite increases towards the rim; Si content in phengite decreases towards the rim. Garnet-glaucophane-phengiteparagonite micaschist is characterised by polymetamorphic garnet porphyroclasts, and small Alpine garnet idioblasts. A pre-Alpine amphibolite-facies metamorphism is inferred for the polymetamorphic rocks of the Châtillon slice. Paragneiss and micaschist probably derive from pre-Alpine "kinzigites"; the orthogneiss protolith was a late-Variscan porphyritic granitoid. Thermobarometry in the eclogite constrains the metamorphic peak at T ≤ 560 °C and P = 16 kbar. The HP minerals were partly retrogressed to greenschist-facies assemblages during the late Alpine tectono-metamorphic recrystallisation. The inferred Alpine P-T conditions are consistent with those for other Penninic and Austro-Alpine nappes of the northwestern internal Alps. The Châtillon slice is very similar to the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone and to the other eclogite-facies Austroalpine slices of the Dent Blanche Nappe, but it could also represent a portion of the Sesia-Lanzo Zone basement, which experienced a somewhat different subduction depth. The tectonic position of the Châtillon slice within the Piemonte Zone is essential to reconstruct the geometric relationships in the Austroalpine-Piemonte nappe stack of the northwestern internal Alps.

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Garnet–chloritoid equilibria in eclogitic pelitic rocks from the Sesia zone (Western Alps): their bearing on phase relations in high pressure metapelites

Cited by 58 publications

References 49 publications

Interplay between deformation and metamorphism during eclogitization of amphibolites in the Sesia–Lanzo Zone of the Western Alps

Interplay between deformation and metamorphism during eclogitization of amphibolites in the Sesia–Lanzo Zone of the Western Alps

The composition and evolution of an Oligocene regolith on top of the Sesia–Lanzo Zone (Western Alps)

Geology and petrology of the Austroalpine Châtillon slice, Aosta valley, western Alps

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