ZIRCON THERMOMETRY AND U-PB ION-MICROPROBE DATING OF THE GABBROS AND ASSOCIATED MIGMATITES OF THE VARISCAN TOLEDO ANATECTIC COMPLEX, CENTRAL IBERIA

Abstract

In the Central Iberian Zone there are several large thermal domes in which small bodies of ultramafic, mafic and intermediate rocks appear intimately associated with crustal granites and migmatites. The closest spatial association between the ultramafic, mafic and intermediate rocks and migmatites is in the Toledo Anatectic Complex, where field relationships suggest that these rocks are coeval and have an age close to 340 Ma. This, and the recent discovery in the neighbouring Ossa Morena Zone of a large mid-crustal seismic reflector interpreted as a 335-350 Ma mafic sill, reinforce the hypothesis that heat for crustal melting was supplied from early Variscan mantle magmas emplaced in the middle crust. However, precise ion-microprobe U-Pb zircon dating and Ti-in-zircon thermometry in Toledo do not support this idea. Whereas the mean age of four mafic bodies is 307 ± 2 Ma, the migmatites are c. 25 Ma older. The migmatites hosting ultramafic, mafic and intermediate bodies have the same age and Ti-in-zircon temperatures as migmatites far from any mafic intrusion. These data reveal that ultramafic, mafic and intermediate magmas are late Variscan; they were emplaced in already cooling anatectic zones once the extensional collapse was initiated, and their thermal impact on the mid-crustal Variscan anatexis of Central Iberia was negligible.