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

摘要

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 meta_morphic 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.