Timescales and mechanisms of batholith construction: constraints from zircon oxygen isotopes and geochronology of the late Variscan Serre Batholith (Calabria, southern Italy)

The late Variscan Serre Batholith in central Calabria represents the middle portion, c. 13-km-thick, of a tiltedcrustal section continuously exposed from lower-crustal granulites to upper-crustal phyllites. The batholith iszoned, consisting of several granitoid types that were emplaced at depths ranging from c. 23 to c. 6 km. Deep,strongly foliated quartz diorites and tonalites were emplaced into migmatitic metapelites, the intermediatelevel granitoids are weakly foliated to unfoliated porphyritic granodiorites and monzogranites, and theshallowest bodies are two-mica granodiorites and granites, grading upward to biotite ± amphibole granodiorites,emplaced into paragneisses and phyllites. Five samples, representative of the main granitoid types interms of both composition and emplacement depth, have been dated by SHRIMP. Zircon from a lower-crustalquartz diorite gave an emplacement age of 297.3 ± 3.1 Ma; two middle-crustal strongly peraluminousK-feldsparmegacrystic graniteswere emplaced at 296.1±1.9Maand 294.9±2.7Ma; a middle- to upper-crustaltwo-mica monzogranite was emplaced at 294.2 ± 2.6 Ma and finally, an upper-crustal weakly peraluminousgranodiorite from the batholith roof was emplaced at 292.2 ± 2.6 Ma. These results are consistent with the developmentof the batholith by incremental multipulse overaccretion, each granitoid body being in a dominantlyrigid state before intrusion of the next, with little or no possibility of magma mixing. The difference in age betweenthe oldest and youngest granitoids, fromthe batholith floor and roof, respectively, is 5.1±4.0Ma, providingan upper limit of about 9Ma on the time taken for batholith construction. The presence in the c. 296–294Magranitoids of c. 305–302Ma anatectic zircon with varied Th/U and oxygen isotope compositions indicates a timeinterval of 8–9 Ma between incipient partial melting in the lower crust and magma emplacement in the middlecrust. The emplacement of the first granitoid bodies into the top of the lower crustwas controlled tectonically bythe activation of a deep-seated shear zone. The shallowest granodiorites were emplaced during the waningstages of the shear zone activity, producing late- to post-tectonic contact metamorphism in the upper-crustalphyllites and mylonitic paragneisses.