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

¹ Institute of Mineralogy, University of Mining and Technology, Brennhausgasse 14, D-09596 Freiberg/Saxony, Germany. Phone: ++49-3731-39-2668; Fax: ++49-3731-39-2610. E-mail: Bernhard.Schulz{at}mineral.tu-freiberg.de

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.

Key Words: Monazite • Garnet zoning • Geothermobarometry • P-T path • Inverted metamorphism • Variscan orogeny • French Massif Central • La Sioule

This article has been cited by other articles:

				J. Melleton, M. Faure, and A. Cocherie Monazite U-Th/Pb chemical dating of the Early Carboniferous syn-kinematic MP/MT metamorphism in the Variscan French Massif Central Bulletin de la Societe Geologique de France, May 1, 2009; 180(3): 283 - 292. [Abstract] [Full Text] [PDF]