Abstract
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.
Highlights
Eclogites exhumed in the core of orogens are commonly considered to evidence ancient subduction zones (e.g. Miyashiro, 1961, 1972; Godard, 2001; Ernst and Liou, 2008)
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
Pioneering works on eclogites in the Massif Central revealed the shape of the retrograde P–T path but the inferred maximum pressures were surprisingly low for a subduction environment
Summary
Eclogites exhumed in the core of orogens are commonly considered to evidence ancient subduction zones (e.g. Miyashiro, 1961, 1972; Godard, 2001; Ernst and Liou, 2008). The discovery of coesite in the Monts du Lyonnais area that typifies UHP metamorphism (Lardeaux et al, 2001) showed that the rocks may record significantly higher pressure This was confirmed by recent studies based on the use of thermodynamic modelling (Berger et al, 2010; Lotout et al, 2018; Lotout et al, 2020). The method used (dissolution of a zircon population) is known to yield erroneous results (Paquette et al, 2017; Lotout et al, 2018) and our work in progress aims at determining the age of the eclogite-facies metamorphism in this locality with modern methods Our focus on this locality for specifying with precision the metamorphic evolution of the eclogite was guided by both the historical importance and the position of the eclogite in the Variscan nappe-stack of the EMC. We take advantage of complex growth/ resorption sequence of porphyroblasts (e.g. garnet, omphacite and amphibole) to decipher rarely identified prograde evolution showing a pressure and temperature increase before an isobaric heating of about 150 °C at peak pressure conditions
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