Two-stage granitoid-forming event from an isotopically homogeneous crustalsource: The Los Pedroches batholith, Iberian Massif, Spain

The Los Pedroches batholith is an aligned series of late Hercynian age plut ons located in the Iberian Massif of Spain, It consists of two main high-k plutonic units: an elongate granodioritic pluton and a group of monzogranit ic plutons trending slightly oblique to the granodiorite. This two-fold dis tinction is supported by several lines of geologic, petrographic, and miner alogical evidence, as well as by contrasting associated mineralization. Che mical variation plots clearly show a compositional gap between the two grou ps, pointing to an origin from two different sets of magmas. This discontin uity is very clear for MgO, CaO, Sr, Zr, rare earth elements, Fe-t/(Fe-t+Mg ), TiO2/Zr, and Eu/Eu* at similar to 70 wt% SiO2 The overall similarity of initial Sr-87/Sr-86,and Nd isotopic ratios, however, suggests that both plu tonic suites were derived from source reservoirs that were similar in terms of time-integrated Rb/Sr and Sm/Nd ratios, In any case, we conclude that n o single-stage process (e.g,, magma mixing) can account for the variation o bserved. Fractional crystallization could theoretically explain the chemica l variation found within each of the granitoid groups. If a single magma se ries is considered, however, modeling of the observed gap requires unlikely scenarios. We suggest that the two sequences were derived from two differe nt parental magma types extracted from isotopically similar sources; each m agma evolved separately by fractional crystallization. On the basis of new isotopic and age data,,ve suggest that these high-K gra nitoids were generated in the lower crust from a mafic to inter: mediate, t ransitional to high-K calc-alkaline, metaigneous source material. Two-stage partial melting of this crustal source material is proposed to explain the evolutionary sequences. Melting could have been triggered by a rise of iso therms related to a transtensional tectonic regime. These granite series sh ow that major contrasts in granite mineralogy, chemistry, and associated mi neral deposits do not necessarily signal major differences in the isotopic signatures Of the corresponding source regions.