Sensibility of tourmaline chemistry to granitic magma composition and oxygen fugacity

Abstract

Massive granitic magmatism is a dominant feature of the Central Iberian Zone (CIZ) of the Variscan Orogen. For the most part, these are S-type per-aluminous granitic rocks exhibiting variable degrees of evolution and compositions ranging from granodiorites, through monzogranites and granites to leucogranites, bearing either biotite and muscovite, or just muscovite in the more evolved facies. Tourmaline is a common and important accessory min-eral in many of these peraluminous granitic rocks. Several granite-hosted tourmaline sets from the Castelo Branco, Idanha-a-Nova and Penamacor-Monsanto plutons were used to investigate how tourmaline chemistry re-flects granitic magma composition and oxygen fugacity. Additionally, previ-ously published data on tourmalines and their host-granites from Rebordelo (CIZ, Portugal) and from the Alamo Complex and several Araya-type granit-ic batholiths (CIZ, Spain) were used to test the trends obtained. Most tour-maline components and component ratios, however, seem essentially imper-vious to granitic magma composition and oxygen fugacity. Exceptions are the Mg/(Mg+Fe) ratio and Ti contents of tourmaline, which show clear vari-ation with the degree of evolution and oxygen fugacity of host granitic rocks. From both mineralogical and petrological point of view, it seems of interest that these compositional features of tourmaline may be used as indi-cators of the degree of evolution and of specific characteristics of the granitic magmas that produced them.