EMP-monazite age controls on P-T paths of garnet metapelites in the Variscan inverted metamorphic sequence of La Sioule, French Massif Central

Abstract

In the La Sioule region (northern French Massif Central), a Variscan inverted metamorphic sequence is made up by high-grade Upper Gneiss (UGU) and Lower Gneiss Units (LGU) which overlie amphibolite-facies micaschists of a Parautochthonous Unit (PAU). Growth-zoned garnets crystallized in gneisses and micaschists and display different Mg-Fe-Mn-Ca evolution trends in the metamorphic units. Microstructurally-controlled geothermobarometry based on cation exchange and net transfer reactions was used to reconstruct syndeformational prograde P-T paths. The P-T paths do not pass eclogite-facies conditions. Different metamorphic peak temperatures confirm the individual character of the evolution in each of the units. EMP-monazite dating (CHIME method) was used to evaluate the age of the P-T-evolution in the garnet-bearing samples. Monazite ages compiled from numerous matrix grains range between 327 and 343 Ma in single samples. Unimodal distributions of ages signalize only slightly younger (320-330 Ma) monazites in the UGU as in the LGU and PAU (330-340 Ma). The inverted metamorphic stratigraphy can be explained by contractional crustal tectonics. In a northern structural domain this involved a top-to-WNW shearing followed by a first thrusting event at the end of garnet growth in UGU and PAU. In a southern domain, the nappe pile was overprinted by a second event with top-to-SE shearing at low pressures. The bulk of monazites should postdate garnet crystallization and the first thrusting event observed in UGU and PAU. At best the~340 Ma monazites in the LGU to the south may have crystallized during the second tectonic event. The bulk monazite crystallization appears to be related to a Late-Visean thermal event when the three units were already juxtaposed in a nappe pile. In consequence, metapelite garnet and monazite crystallization in the La Sioule series belong to a Late Devonian -Early Carboniferous prograde-retrograde metamorphism. This metamorphic evolution appears as a second metamorphic cycle subsequent to the Silurian -Early Devonian HP and HT events within the complex frame of the Variscan orogeny in the French Massif Central. Contrôle sur l´âge des chemins P-T en métapelites à grenat par datation EMP de la monazite dans la série du métamorphisme inverse hercynienne de La Sioule, Massif central francaisMots-clés. -Monazite, Zonation du grenat, Géothermobarometrie, Chemin P-T, Métamorphisme inverse, Chaîne varisque, Massif Central, La Sioule.Résumé. -Dans la région de la Sioule (Massif central français) une série métamorphique inverse est formée par une unité supérieure des gneiss (UGU), et une unité inférieure des gneiss (LGU) de haut grade qui surmonte les micaschistes du faciès amphibolite de l'unité para-autochtone (PAU). Des grenats zonés cristallisent dans les gneiss et les micaschistes et présentent des évolutions de concentrations en Mg-Fe-Mn-Ca contrastées. La géothermobarométrie basée sur les échanges cationiques et les réactions de transfert a été appliquée à des minéraux replacés dans leur contexte micro...

Geochemical Signature of Magmatic-Hydrothermal Fluids Exsolved from the Beauvoir Rare-Metal Granite (Massif Central, France): Insights from LA-ICPMS Analysis of Primary Fluid Inclusions

“…The Sioule metamorphic series is composed by three lithostructural units, which are from top to bottom [34]: (i) an upper unit of cordierite-bearing migmatite and biotite-sillimanite gneiss, corresponding to the UGU; (ii) an intermediate unit composed by biotite-muscovite paragneiss, corresponding to the LGU; and (iii) a lower unit formed by biotite-muscovite micaschists, which is correlated with the PAU. This series experienced two major synmetamorphic events [35,36]: (i) a HP D 0 event coeval with eclogitic-granulitic metamorphism at ca. 430-400 Ma; and (ii) a MP-MT D 2 event coeval with the top-to-the-NW shearing at ca.…”

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

“…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). …”

“…One can relate the mineral chemical data from biotite and plagioclase to the garnet compositional evolution for the early and late stages of crystallization. This corresponds to criteria for the definition of local equilibria based on microstructural observations and the chemical evolution of the mineral phases (Triboulet and Audren 1985;Schulz 1993;Schulz et al 2001). Such localised equilibria within low-variance assemblages allow evaluating P-T changes for the consecutive stages of garnet growth by geothermobarometry (Spear 1993).…”

“…The dependence of the calculated P-T path on garnets X Ca and X Mg evolution is intensified when corresponding micas and plagioclases have only slight compositional variations, or when X Mg in biotite and X Ca in plagioclase evolved oppositely to the corresponding X Mg and X Ca variations in garnets. Shapes or the relative P/∆T trends of P-T paths appear to be mainly preserved when the uncertainties regarding corresponding plagioclase and biotite are considered (Schulz et al 2001). Due to the preserved high-Mn cores in small garnet porphyroblasts, a high-temperature homogenisation by cation diffusion with its effects on geothermobarometry (Spear 1993) can be excluded.…”

“…These findings match data from comparable terrains in the French Massif Central (Bellot and Roig 2007). Especially from the La Sioule region to the north were reported kyanite garnet gneisses with comparable microstructures, mineral assemblages, garnet Mg-Ca zonation trends, P-T path conditions and shapes, and monazite ages (Schulz et al 2001;Schulz 2009). Thus, the P-T data confirm the idea of an inverted metamorphic gradient in the French Massif Central.…”