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

The Jalama pluton (JP) is one of the best examples of the late Hercynian tw o-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 fac ies [a two-mica coarse-grained porphyritic granite (PG), a two-mica coarse- grained equigranular granite (EG) and a tourmaline-bearing-muscovite leucog ranite (TL)] which define both vertical and horizontal zonation, The main p etrographic 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 mon zogranite (SM) is distinguished by the presence of micaceous aggregates com posed of biotite, muscovite and sillimanite. Based on major and trace eleme nt data the compositional variability within the SM has been ascribed to in heritance from compositionally heterogeneous source rocks; (3) the late leu cogranites (LL) appear mainly as dykes intruded in the PG. From a compariso n with this PG, it can be deduced that the LL represent derivative liquids segregated after important fractionation from magmas equivalent to PG magma s but at deeper levels. The most important implication of our study is that the Jalama granites were generated by partial melting of high-grade metamo rphic peraluminous rocks that were compositionally heterogeneous on differe nt scales. During this small-scale intracrustal melting event, temperature was the limiting factor for the degree of melting. (C) 2000 Elsevier Scienc e B.V. All rights reserved.