The Mg/(Fe + Mg) ratio and the Ti and A site contents of tourmaline as promising indicators of granitic magma evolution

Abstract

In the recent past, important research has shown how tourmaline can be used as a petrogenetic indicator in pegmatite rocks and related mineralization. Previous studies have demonstrated that tourmaline composition may be a useful guide to ascertain the degree of evolution and oxygen fugacity of host granitic rocks from the Variscan orogen in the Central Iberian Zone (CIZ). Following those promising preliminary conclusions, we have investigated whether these results hold for tourmaline-bearing granitic rocks from other geotectonic contexts, enlarging our scope to include whole-rock stable isotope analyses. The data collected so far has shown that, whereas most tourmaline components and component ratios seem essentially impervious to granitic magma composition and oxygen fugacity, there are a few exceptions, such as the Mg/(Mg + Fe) ratio and Ti contents of tourmaline that show clear variation with the degree of evolution and oxygen fugacity of the host granitic rocks. From both mineralogical and petrological points of view, it seems that these compositional features in tourmaline may be used as indicators of the degree of evolution and of specific characteristics of the granitic magmas that produced them.