Ordovician tectonics and crustal evolution at the Gondwana margin (Central Iberian Zone)

Abstract: U-Pb ages of zircon from samples of tonalite and migmatite from the Central Iberian Zone (Spain), previously assigned to the Carboniferous were obtained using a sensitive high mass-resolution ion microprobe. These ages indicate a close temporal relationship between Ordovician tonalitic magmatism and high-grade (low-P) metamorphism in Iberia. The age obtained for Santa Cruz tonalite (476  ± 1 Ma) represents a constraint on the partial melting event experienced by Ediacaran Schist-Greywacke Complex siliciclastic rocks, while Risco Chico leucogranite that alternates with tonalites and migmatites yielded a younger age (468  ± 3 Ma). The age range of Neoproterozoic relict zircon grains preserved in Ordovician plutonic rocks coincides with that of host Ediacaran metasedimentary rocks (Puerto de Santa Cruz migmatites), which are the most plausible source of leucogranites. Altogether, the component of inherited zircon, peraluminous calc-alkaline composition, and an abundance of pelitic restites found in Ordovician tonalites provide evidence of a mixed contribution from old Ediacaran magmatic arc rocks and siliciclastic sequences. Our findings supply a missing piece in the Lower-Middle Ordovician tectonic puzzle of the northern Gondwana margin, revealing that tonalitic magmatism, ductile deformation, and high-grade metamorphism all occurred within a relatively restricted period simultaneously with siliciclastic deposition in active extensional basins.