Causes of geochemical diversity in peraluminous granitic plutons: the Jálama pluton, Central-Iberian Zone (Spain and Portugal)

Abstract

The Jálama pluton (JP) is one of the best examples of the late Hercynian two-mica granites of the Central-Iberian Zone (CIZ) of the Hesperian Massif (Spain and Portugal). This pluton is a multiphase intrusion that consists of three intrusive units: (1) the external unit (EU) comprises three main facies [a two-mica coarse-grained porphyritic granite (PG), a two-mica coarse-grained equigranular granite (EG) and a tourmaline-bearing-muscovite leucogranite (TL)] which define both vertical and horizontal zonation. The main petrographic and compositional characteristics of the EU can be explained by a fractional crystallization process that takes place in independent magma batches generated from heterogeneous source rocks; (2) the sillimanite monzogranite (SM) is distinguished by the presence of micaceous aggregates composed of biotite, muscovite and sillimanite. Based on major and trace element data the compositional variability within the SM has been ascribed to inheritance from compositionally heterogeneous source rocks; (3) the late leucogranites (LL) appear mainly as dykes intruded in the PG. From a comparison with this PG, it can be deduced that the LL represent derivative liquids segregated after important fractionation from magmas equivalent to PG magmas but at deeper levels. The most important implication of our study is that the Jálama granites were generated by partial melting of high-grade metamorphic peraluminous rocks that were compositionally heterogeneous on different scales. During this small-scale intracrustal melting event, temperature was the limiting factor for the degree of melting.