Chemical variability of Al-Ti-Fe-Mg minerals in peraluminous granitoid rocks from Central Spain

Abstract

Two broad series of peraluminous granite types can be distinguished in the Central System of Spain. The first group consists of cordierite-bearing granites that are moderately peraluminous in composition (peraluminous S-type granites, PS). The second is composed of amphibole/allanite-bearing granites, weakly peraluminous in composition (peraluminous I-type, PI). These granite types evolved through complex igneous fractionation processes. Both peraluminous granite types show differences in the chemical composition of biotite and cordierite; cordierite is common in PS granites but restricted to some highly fractionated PI granites. Garnet and ilmenite cannot be used as chemical criteria for distinguishing the two series. The Al and Na content of biotite and cordierite correlate well with the granite type (PS and PI). The higher Na content in biotite (and cordierite) of PS granites is noteworthy. When plotted in a Na vs. [VI1A1 diagram biotites are located in different fields according to the peraluminosity degree of the granite type. The estimated Fe3+/(Fe3++Fe2+) ratio of biotite is higher in PI than in PS granites, although this value is very pluton-dependent. Several compositional variations of Al-Ti-Fe-Mg (ATFM) minerals are functions of the degree of fractionation or evolution of the magma, irrespectively of its peraluminosity. Other chemical features of ATFM phases, such as XFe or XMΠ, can be used as fractionation indices. Thus, the more acid the magma, the higher XFe and XMΠ in ATFM minerals. This correlation is not linear showing a rapid increase for silica values of SiO2 ≥ 74 %. Usually, XFe garnet > XFe biotite > XFe cordierite, and XMΠ ilmenite = XMΠ garnet > XMΠ cordierite > XMΠ biotite, while XFe and XMΠ of whole rock are very close to biotite values. The high increase in Mn content, more marked than in Fe, in all AFM phases in the silica-rich granites (SiO2 = 74 %), can be interpreted as a consequence of the incompatible behaviour of this element in highly evolved granites, that are sufficiently poor in Ti, Mg and Fe to prevent or diminish crystallization of ore-minerals, such as ilmenite.