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

The Los Pedroches batholith is an aligned series of late Hercynian age plutons located in the Iberian Massif of Spain. It consists of two main high-K plutonic units: an elongate granodioritic pluton and a group of monzogranitic plutons trending slightly oblique to the granodiorite. This twofold distinction is supported by several lines of geologic, petrographic, and mineralogical evidence, as well as by contrasting associated mineralization. Chemical variation plots clearly show a compositional gap between the two groups, pointing to an origin from two different sets of magmas. This discontinuity is very clear for MgO, CaO, Sr, Zr, rare earth elements, Fe_t/(Fe_t+Mg), TiO₂/Zr, and Eu/Eu* at ∼70 wt% SiO₂. The overall similarity of initial ⁸⁷Sr/⁸⁶Sr and Nd isotopic ratios, however, suggests that both plutonic 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 no single-stage process (e.g., magma mixing) can account for the variation observed. Fractional crystallization could theoretically explain the chemical variation found within each of the granitoid groups. If a single magma series is considered, however, modeling of the observed gap requires unlikely scenarios. We suggest that the two sequences were derived from two different parental magma types extracted from isotopically similar sources; each magma evolved separately by fractional crystallization.

On the basis of new isotopic and age data, we suggest that these high-K granitoids were generated in the lower crust from a mafic to intermediate, transitional 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 isotherms related to a transtensional tectonic regime. These granite series show that major contrasts in granite mineralogy, chemistry, and associated mineral deposits do not necessarily signal major differences in the isotopic signatures of the corresponding source regions.