Effects of diffusional modification of garnet growth zoning on P-T path calculations

1991 ◽  
Vol 107 (4) ◽  
pp. 487-500 ◽  
Author(s):  
Frank P. Florence ◽  
Frank S. Spear
Keyword(s):  
Garnet Growth ◽  
T Path ◽  
Garnet Growth Zoning ◽  
Growth Zoning

Documenting isobaric cooling in the lower crust using cordierite breakdown textures (Mont Mary nappe, Western Alps)

2021 ◽  
Author(s):  
Michel Ballèvre ◽  
Marc Poujol ◽  
Selim Rousseau ◽  
Paola Manzotti
Keyword(s):  
Western Alps ◽  
Detrital Zircons ◽  
Garnet Growth ◽  
Garnet Porphyroblasts ◽  
Chemical Zoning ◽  
Lower Unit ◽  
The Matrix ◽  
T Path ◽  
The One ◽  
Growth Zoning

<p>Intracrystalline diffusion is an efficient mechanism in high-grade rocks. Therefore, growth zoning in garnet is erased and the evidence for prograde path is lost. However, information recorded by the textures may store significant clues for deciphering part of the P-T path. An example is provided here from the migmatitic paragneisses from the Mont Mary nappe (Western Alps).</p><p>The latter is made of a pre-Alpine basement consisting of an upper and a lower unit. The upper unit is made of paragneisses, marbles and amphibolites similar to those of the Valpelline Unit and of the Ivrea Zone. The lower unit displays granitic orthogneisses, paraschists (with muscovite, biotite, garnet with local occurrences of staurolite, kyanite and andalusite) (Dal Piaz et al. 2015). In this unit, we discovered a hectometre-sized volume with no Alpine overprint, preserving migmatitic paragneisses, the topic of this study.</p><p>The paragneisses display quartzo-feldspathic leucocratic layers interpreted as crystallized melts. The leucosomes are separated by biotite- and sillimanite-rich layers, with conspicuous garnet porphyroblasts. In addition, fresh cordierite crystals are found in these layers. Sillimanite included in garnet rims has the same orientation than the one in the matrix. There, the foliation is defined by the shape fabric of biotite and sillimanite, wrapping both garnet and cordierite crystals.</p><p>Such textures may be used to propose a P-T path. A sequence of prograde reactions, including dehydration-melting of muscovite, then biotite, result in the production of a large amount of sillimanite. Garnet growth was continuing during incongruent melting. However, intracrystalline diffusion has erased the prograde chemical zoning, as well as the distribution and shape of mineral inclusions. The late replacement of garnet and cordierite by biotite and sillimanite indicates near-isobaric cooling, also recorded by chemical zoning along garnet rims.</p><p>Chemical data on coexisting minerals will be used to provide quantitative constraints on the P-T path. In addition, preliminary geochronological data suggest that detrital zircons grains were significantly reset during the HT metamorphism, which could have taken place c. 270 Ma ago. To conclude, the studied paragneisses offer another example of Permian near-isobaric cooling in the middle crust of the Adriatic plate.</p><p>Dal Piaz G.V., Bistacchi A., Gianotti F., Monopoli B., Passeri L., Schiavo A. & collaboratori (2015) – Note illustrative della carta Geologica d’Italia alla scala 1:50.000. Foglio 070, Monte Cervino. ISPRA, Servizio Geologico d’Italia, 070, 1-431.</p>

Well‐preserved garnet growth zoning in granulite from the Dabie Mountains, central China

1998 ◽  
Vol 16 (2) ◽  
pp. 213-222 ◽  
Author(s):  
NENG‐SONG CHEN ◽  
MIN SUN ◽  
ZHEN‐DONG YOU ◽  
J. MALPAS
Keyword(s):  
Central China ◽  
Garnet Growth ◽  
Dabie Mountains ◽  
Garnet Growth Zoning ◽  
Growth Zoning

scholarly journals Prograde and retrograde P–T evolution of a Variscan high-temperature eclogite, French Massif Central, Haut-Allier

2020 ◽  
Vol 191 ◽  
pp. 14
Author(s):  
Luc de Hoÿm de Marien ◽  
Pavel Pitra ◽  
Florence Cagnard ◽  
Benjamin Le Bayon
Keyword(s):  
High Pressure ◽  
High Temperature ◽  
Garnet Growth ◽  
French Massif Central ◽  
Temperature Peak ◽  
Massif Central ◽  
Slab Rollback ◽  
High Pressure Granulite ◽  
Garnet Growth Zoning ◽  
Growth Zoning

The P–T evolution of a mafic eclogite sample from the Haut-Allier was studied in order to constrain the dynamic of the Variscan subduction in the eastern French Massif Central. Three successive metamorphic stages M1, M2 and M3, are characterized by assemblages comprising garnet1-omphacite-kyanite, garnet2-plagioclase, and amphibole-plagioclase, respectively, and define a clockwise P–T path. These events occurred at the conditions of eclogite (M1; ∼ 20 kbar, 650 °C to ∼ 22.5 kbar, 850 °C), high-pressure granulite (M2; 19.5 kbar and 875 °C) and high-temperature amphibolite facies (M3; < 9 kbar, 750–850 °C), respectively. Pseudosection modelling of garnet growth zoning and mineralogy of the inclusions reveal a prograde M1 stage, first dominated by burial and then by near isobaric heating. Subsequent garnet1 resorption, prior to a renewed growth of garnet2 is interpreted in terms of a decompression during M2. High-pressure partial melting is predicted for both the M1 temperature peak and M2. M3 testifies to further strong decompression associated with limited cooling. The preservation of garnet growth zoning indicates the short-lived character of the temperature increase, decompression and cooling cycle. We argue that such P–T evolution is compatible with the juxtaposition of the asthenosphere against the subducted crust prior to exhumation driven by slab rollback.

scholarly journals Preservation of Garnet Growth Zoning and the Duration of Prograde Metamorphism

2010 ◽  
Vol 51 (11) ◽  
pp. 2327-2347 ◽  
Author(s):  
Mark J. Caddick ◽  
Jiří Konopásek ◽  
Alan B. Thompson
Keyword(s):  
Garnet Growth ◽  
Prograde Metamorphism ◽  
Garnet Growth Zoning ◽  
Growth Zoning
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