Temporal relationships between Mg-K mafic magmatism and catastrophic melting of the Variscan crust in the southern part of Velay Complex (Massif Central, France)

Abstract

Mg-K mafic intrusive rocks are commonly observed during the late stages of the evolution of orogenic belts. The Variscan French Massif Central has many outcrops of these rocks, locally called vaugnerites. Such magmas have a mantle-derived origin and therefore allow discussion of the role of mantle melting and crust-mantle interactions during late-orogenic processes. In the Southern Velay area of the French Massif Central, LA-ICPMS U-Pb dating on zircons and monazites from three vaugnerites and four coeval granites reveals that the two igneous suites formed simultaneously, at c. 305 Ma. This major igneous event followed after an early, protracted melting stage that lasted for 20-30 My and generated migmatites, but the melt was not extracted efficiently and therefore no granite plutons were formed. This demonstrates that widespread crustal anatexis, melt extraction and granite production were synchronous with the intrusion of vaugneritic mantle-derived melts in the crust. The rapid heating and subsequent melting of the crust led to upward flow of partially molten rocks, doming and collapse of the belt.

Pre-Cadomian to late-Variscan odyssey of the eastern Massif Central, France: Formation of the West European crust in a nutshell

“…3, 5), interpreted as the age of crystallization of the Velay granite at Tournon. These ages are consistent with other geochronological data obtained from the Velay granites-migmatites, all yielding ages in the range 300-310 Ma (Couzinié et al, 2014;Didier et al, 2013;Laurent et al, 2017;Mougeot et al, 1997). In the LGU migmatitic paragneiss TN17, the age of the migmatization is well recorded by zircon rims on older cores or entire grains (307.4 ± 2.3 Ma) and overlap within uncertainty with the age of the Velay granite as dated on TN01 and TN09 (Fig.…”

“…also notably similar to that observed from other Variscan granitoids in the area (Couzinié et al, 2014;Laurent et al, 2017;Moyen et al, 2016).…”

“…It consists of orthogneisses (peraluminous metagranite and metarhyolite), paragneisses, and minor amphibolites with dominantly Cambrian protoliths (Caen-Vachette, 1979;Mintrone, 2015;R'Kha Chaham et al, 1990). Following amphibolite facies metamorphism, rocks of the LGU were affected by a major late Carboniferous HT-LP event, marked by widespread crustal melting that led to the formation of the Velay granite-migmatite dome (Barbey et al, 1999;Burg and Vanderhaeghe, 1993;Couzinié et al, 2014;Dupraz and Didier, 1988;Gardien et al, 1997;Ledru et al, 2001;Montel et al, 1992;Vanderhaeghe et al, 1999). In contrast to the UGU, the LGU does not record a HP event in the EMC.…”

“…The youngest dike (TN32; 307.9 ± 3.3 Ma) is concomitant with the emplacement of the Velay granites, dated at 309.6 ± 2.9 Ma (TN01) and 307.5 ± 2.0 Ma (TN09) in Tournon and in the 310-300 Ma time span elsewhere in the EMC (Couzinié et al, 2014;Didier et al, 2013;Laurent et al, 2017;Mougeot et al, 1997). Overall, data from the dikes argue for the presence of a 15.4 ± 4.8 Ma-long lasting thermal anomaly in the Tournon area, accompanied with protracted crustal partial melting and granite dike intrusion.…”

“…However, this magmatism was spatially and temporally associated with significant input of mantle-derived magmas (e.g., biotite-amphibole calc-alkaline granodiorites, quartz-diorites and high-Mg and high-K mafic rocks; Fig. 1; Barbey et al, 1999Barbey et al, , 2015Couzinié et al, 2014Couzinié et al, , 2016Downes et al, 1997;Laurent et al, 2017;Solgadi et al, 2007;Williamson et al, 1996;. The formation of the Velay granite-migmatite dome at 310-300 Ma marks the gravitational collapse of the orogen, which was coeval with the development of ductile extensional shear zones and opening of the late Carboniferous (Stephanian) coal-bearing basins (Burg et al, 1990;Burg and Vanderhaeghe, 1993;Malavieille, 1993;Malavieille et al, 1990).…”