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 peraluminous granitoids that were preceded by volumetrically insignificant bodies of mafic‐ultramafic hybrid magmas and postdated by several dike swarms of camptonitic lamprophyres. Rb‐Sr dating indicates continuous magmatic activity from ~ 350 Ma to ~ 280 Ma, starting with the mafic precursors and a few midcrustal anatectic leucogranites, followed by massive autochthonous and allochthonous granodiorites and granites, and ending with the camptonitic lamprophyres. Early hybrid mafic magmas (ε 340\:Ma\,\,Sr\,~ 25 ; ε 340\:Ma\,\,Nd\,~ -1.5 ) were produced in small batches during or immediately after the main deformation phase, probably by the partial melting of a mixture of ~ 8 : 2 mantle and biotite‐bearing crustal rocks at the crust‐mantle interface. These magmas were emplaced in the middle crust at ~ 340 Ma, advecting a negligible amount of heat. The generation of crustal granites during the main deformation phase was very scarce and limited to highly fertile protoliths, rich in heat‐producing elements, affected by strong shear zones. The generation of crustal granitoids on a batholithic scale took place from ~ 330 Ma to ~ 290 Ma, during the main extensional period. Granites (ε 310\:Ma\,\,Sr\,~ 45 –150; ε 310\:Ma\,\,Nd\,~ -2.1\,to\,\,-9 ) were produced by the partial melting of fertile crustal rocks (ε 310\:Ma\,\,Sr\,~ 48 –218; ε 310\:Ma\,\,Nd\,~ -2.2\,to\,\,-9 ), characterized by high heat production (~ 2.5–3 μW m−3). The zone of partial melting, ~ 15–22 km in depth, was heated by thermal conduction from below 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 ~ 310 Ma to ~ 285 Ma released lithostatic pressure in the upper mantle and caused decompressional melting of the metasome layer at ~ 60–85 km in depth, producing camptonitic melts dated at ~ 283 Ma. The existence of a fertile metasome layer implies that the lithospheric mantle beneath central Iberia was not actively involved in subduction during the Variscan orogeny.