From crustal anatexis to mantle melting in the Variscan orogen of Corsica (France): SIMS U-Pb zircon age constraints

Overview & Synopsis

High-precision SIMS U-Pb zircon age determinations are conducted in this study on migmatites and Mg-K magmatic suites from the Variscan orogen of Corsica (France). Zircons from leucosomes of four migmatites yield consistent crystallization ages of ca. 345 Ma. Four Mg-K granitoid rocks and one monzogabbro enclave from the Calvi-IIe Rousse pluton of NW Corsica yield indistinguishable U-Pb zircon ages of ca. 330 Ma. These new SIMS zircon U-Pb dating results indicate that the regional crustal anatexis under amphibolite-facies condition occurred synchronously at ca. 345 Ma throughout the Corsican Variscan chain. There is a ca. 15 m.y. time interval between anatexis of the thickened crust and partial melting of the metasomatized mantle and overlying crust to form the Mg-K rock suites. In combination with the previous dating results for the high-temperature and high-pressure granulite-facies metamorphism, three major discrete tectonothermal events at ca. 360 Ma, ca. 345 Ma, and ca. 330 Ma are acknowledged. We argue for a slab break-off model accounting for thermal and mechanical evolution of the crust within the Variscan orogen of Corsica. A tearing of the subducting oceanic lithosphere initiates the asthenosphere rise to fill the void and causes the granulite-facies metamorphism of the overlying continental crust at ca. 360 Ma. Final breaking of the subducting oceanic lithosphere results in exhumation of the subducted crust to the thickened lower to middle crustal-level. Internal radiogenic heating causes crust-scale migmatization at ca. 345 Ma. Slab break-off and foundering enhance the rise of hot asthenosphere, resulting in conducted thermal perturbation that leads to melting of the metasomatised enriched mantle lithosphere to form Mg-K mafic-granitoid magmatism at ca. 330 Ma.