Origin of peraluminous granites and granodiorites, Iberian massif, Spain: an experimental test of granite petrogenesis

The discrimination between potential source materials involved in the genes is of Iberian granites and granodiorites, as well as the role of mantle-cru st interactions, are examined using constraints imposed by melting experime nts, melting-assimilation experiments and Sr-Nd isotope systematics. The Sr -Nd isotope relationships indicate the existence of different genetic trend s in which juvenile mantle materials are involved by different mechanisms: (1) a source trend, traced by a particular evolution of the pre-Hercynian b asement and indicating mantle participation at the time of sedimentation; ( 2) a set of magmatic trends traced by gabbrotonalite-enclave-granodiorite a ssociations, implying the incorporation of new mantle material at the time of granite generation. These relationships strongly support a pure crustal origin for the peraluminous leucogranites, derived from partial melting of crustal protoliths, and a hybrid origin for the peraluminous granodiorites. These granodiorites are the most abundant granitic rocks of the Central Ib erian zone (CIZ) of the Iberian massif, implying that processes of hybridis ation by assimilation and/or magma mixing played an important role in grani toid production during the Hercynian orogeny. These hypotheses have been te sted by means of melting and assimilation experiments. Melting experiments in the range 800-900 degrees C and at pressures of 3, 6, 10 and 15 kbar ind icate that: (1) several potential source materials such as Bt-Ms gneisses a nd metagreywackes are suitable for the production of peraluminous leucogran ite melts; (2) the melt compositions are always leucogranitic, regardless o f pressure; (3) pressure exerts a strong influence on the fertility of the source: experiments at 3 kbar produce more than 20 vol% of melt, compared w ith less than 5 vol% of melt produced at 10 and 15 kbar and at the same tem perature. The melting-assimilation experiments carried out at 1000 degrees C and 4, 7 and 10 kbar and using a proportion of 50% gabbro and 50% gneiss give high melt proportions (more than 50 vol.%) and noritic residues. These melts have the composition of leucogranodiorites, and overlap with part of the compositional range of peraluminous granodiorites of the Iberian massi f. The generation of more mafic granodiorites may be explained by the incor poration of some residual orthopyroxene to the granodiorite magmas. The low solubility of Fe + Mg prevents the generation of granodiorite melts with m ore than 3 wt% of MgO + FeO at all crustal pressures. The large volumes of peraluminous, hybrid granodiorites, produced by assimilation of crustal roc ks by mantle magmas, imply that an important episode of crustal growth took place during the Late-Palaeozoic Hercynian orogeny in the Iberian massif.