Structure

Ledru, Patrick; Costa, Sylvie; Echtler, Helmut

doi:10.1007/978-3-642-84915-2_27

Cited by 28 publications

(7 citation statements)

References 29 publications

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“…Thus, the P-T data confirm the idea of an inverted metamorphic gradient in the French Massif Central. It furthermore supports the concept of a maximal allochthony of the UGU (Burg and Matte 1978; Ledru et al 1989Ledru et al , 1994. The metamorphic evolution of the UGU at Agnat was syntectonic, as documented by inclusion trails S 1 i in garnet.…”

Section: Conclusion and Implications To The Variscan Metamorphic Evosupporting

confidence: 75%

“…These nappes overlie the Para-Autochthonous units, which are mainly composed of quartzites, micaschists and scarce Ordovician orthogneisses. No early high-pressure relics are observed in the Para-Autochthonous units, which variably underwent lower and upper amphibolite-facies metamorphism (Ledru et al 1989(Ledru et al , 1994Quenardel et al 1991;Santallier et al 1994;Faure et al 2005Faure et al , 2009. The thrusting of the UGU over lower grade units is considered to be responsible for an inverted metamorphic stratigraphy ("inverted metamorphism") which is widespread and typical of several regions in the French Massif Central, such as the Aigurande (Rolin and Quenardel 1980), Marvejols (Briand 1978), Rouergue (Burg et al 1989), Limousin (Friedrich et al 1988;Roig and Faure 2000;Bellot and Roig 2007), Haut-Allier (Marchand 1974;Burg 1977;Burg et al 1984;Schulz et al 1996) and La Sioule (Faure et al 1993;Schulz et al 2001;Schulz 2009).…”

Section: Geological Setting and Sampling Sitesmentioning

confidence: 99%

“…1c). In several geological sketch maps, these micaschists are assigned to the LGU (Ledru et al 1989(Ledru et al , 1994Faure et al 2009). However, they show different lithology, mineral assemblages and metamorphism in the Massiac-Mercoeur anticline.…”

Section: Geological Setting and Sampling Sitesmentioning

confidence: 99%

“…Five principal nappe units (from top to bottom: Brévenne and Génis, Thiviers-Payzac, Upper Gneiss, Lower Gneiss, and Para-Autochthonous units) have been identified in the internal part of the French Massif Central, as outlined in Ledru et al (1994) and Faure et al (2005Faure et al ( , 2009. Some of the allochthonous units (Fig.…”

Section: Introductionmentioning

confidence: 95%

“…This nappe pile was thrusted over an unknown Upper Proterozoic basement with a Pre-Carboniferous cover in some parts (Faure et al 2009). Some of the nappes, especially the els (Brun and Burg 1982;Burg et al 1987;Ledru et al 1994;Matte 2001) involved a single P-T-t path for all tectonic units during a long and progressive subduction, followed by exhumation of the HP rocks and a collision with migrating directions of thrusting. This paper reports detailed chemical model (CHIME) Th-U-Pb monazite ages and geothermobarometric data of a prograde-retrograde P-T path section from a kyanite garnet gneiss in the Upper Gneiss Unit of the Haut-Allier area.…”

Section: Introductionmentioning

confidence: 99%

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Early Carboniferous P-T path from the Upper Gneiss Unit of Haut-Allier (French Massif Central) - reconstructed by geothermobarometry and EMP-Th-U-Pb monazite dating

Schulz¹

2014

Jour. Geosci.

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In the Haut-Allier region (French Massif Central), a Variscan inverted metamorphic sequence is made up by crustal nappes. A high-grade Upper Gneiss Unit (UGU) was thrusted over a Lower Gneiss Unit (LGU) and an amphibolite--facies para-autochthonous Micaschist Unit (MU). Growth-zoned garnets with distinct Mn-Mg-Fe-Ca and trace-element zonation trends occur in kyanite garnet gneisses at Agnat (UGU). The porphyroblasts have been characterised by automated energy-dispersive X-ray spectral mapping with SEM, by electron microprobe and LA-ICPMS analyses. Microstructurally-controlled geothermobarometry based on cation exchange and net transfer reactions was used to reconstruct a syn-deformational and clockwise P-T path for garnet crystallization in the UGU. The P-T path passed maximal pressures at 700 °C/13 kbar and then maximal temperatures at ~800 °C/11 kbar in the stability field of kyanite + K-feldspar. The final stage of the P-T path is a marked decompression from 10 to 5 kbar at 700-750 °C. The timing of this P-T evolution in the UGU has been constrained by electron-microprobe Th-U-Pb monazite dating (CHIME). A detailed interpretation of ages and Y-contents of monazites enclosed in a syncrystalline-rotated Mg-rich garnet allowed to relate a marked pressure decrease along a late stage of the P-T path to ~330 Ma. Simple 1D forward numerical modeling with variations of vertical velocity and geothermal gradient confirmed that the onset of monazite crystallization at ~360 Ma should give a maximal age of the early P-T evolution. A distinct group of Y-rich monazites could be the relic of this Late Devonian event. Most monazite ages from the UGU, the LGU and the MU in the Haut-Allier region range between 360 and 320 Ma, with isochrons for single samples giving ages from 332 to 338 Ma. The prograde-retrograde P-T-t evolution in the UGU appears as an independent Early Carboniferous metamorphic cycle, which was related to a continental collision. It post-dated a Silurian HP-(UHP) event and a subsequent Early Devonian migmatization in the UGU.

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Section: Conclusion and Implications To The Variscan Metamorphic Evosupporting

confidence: 75%