Mafic precursors, peraluminous granitoids, and late lamprophyres in the Avila batholith: A model for the generation of Variscan batholiths in Iberia

The Avila batholith of central Spain is composed of upper Carboniferous per aluminous granitoids that were preceded by volumetrically insignificant bod ies of mafic-ultramafic hybrid magmas and postdated by several dike swarms of camptonitic lamprophyres. Rb-Sr dating indicates continuous magmatic act ivity from similar to 350 Ma to similar to 280 Ma, starting with the mafic precursors and a few midcrustal anatectic leucogranites, followed by massiv e autochthonous and allochthonous granodiorites and granites, and ending wi th the camptonitic lamprophyres. Early hybrid mafic magmas (epsilon(340Ma) Sr similar to 25; epsilon(340Ma) Nd similar to -1.5) were produced in small batches during or immediately after the main deformation phase, probably b y the partial melting of a mixture of similar to 8 :2 mantle and biotite-be aring crustal rocks at the crust-mantle interface. These magmas were emplac ed in the middle crust at similar to 340 Ma, advecting a negligible amount of heat. The generation of crustal granites during the main deformation pha se was very scarce and limited to highly fertile protoliths, rich in heat-p roducing elements, affected by strong shear zones. The generation of crusta l granitoids on a batholithic scale took place from similar to 330 Ma to si milar to 290 Ma, during the main extensional period. Granites (epsilon(310M a) Sr similar to 45-150; epsilon(310Ma) Nd similar to-2.1 to -9) were produ ced by the partial melting of fertile crustal rocks (epsilon(310Ma) Sr simi lar to 48-218; epsilon(310Ma) Nd similar to 2.2 to -9), characterized by hi gh heat production (similar to 2.5-3 mu W m(-3)). The zone of partial melti ng, similar to 15-22 km in depth, was heated by thermal conduction from bel ow after crustal thinning, but the contribution of radiogenic heat and the fertility of source rocks would have been essential for anatexis. The fast thinning of the crust from similar to 310 Ma to similar to 285 Ma released lithostatic pressure in the upper mantle and caused decompressional melting of the metasome layer at similar to 60-85 km in depth, producing camptonit ic melts dated at similar to 283 Ma. The existence of a fertile metasome la yer implies that the lithospheric mantle beneath central Iberia was not act ively involved in subduction during the Variscan orogeny.