CHEMICAL VARIABILITY OF AL-TI-FE-MG MINERALS IN PERALUMINOUS GRANITOID ROCKS FROM CENTRAL SPAIN

Two broad series of peraluminous granite types can be distinguished in the Central System of Spain. The first group consists of cordierite-b earing granites that are moderately peraluminous in composition (peral uminous S-type granites, PS). The second is composed of amphibole/alla nite-bearing granites, weakly peraluminous in composition (peraluminou s I-type, PI). These granite types evolved through complex igneous fra ctionation processes. Bath 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 gr anites. Garnet and ilmenite cannot be used as chemical criteria for di stinguishing the two series. The Al and Na content of biotite and cord ierite correlate well with the granite type (PS and PI). The higher Na content in biotite (and cordierite) of PS granites is noteworthy. Whe n plotted in a Na vs. Al-[VI] diagram biotites are located in differen t fields according to the peraluminosity degree of the granite type. T he estimated Fe3+/Fe3++Fe2+) ratio of biotite is higher in PI than in PS granites, although this value is very pluton-dependent. Several com positional 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 X(Fe) or X(Mn), can be used as fractionation indices. Thus, the more acid the magma, the higher X(Fe) and X(Mn) in ATFM minerals. This corr elation is not linear showing a rapid increase for silica values of Si O2 greater than or equal to 74 %. Usually, X(Fe) garnet > X(Fe) biotit e > X(Fe) cordierite, and X(Mn) ilmenite congruent to X(Mn) garnet 2 X (Mn) cordierite > X(Mn) biotite, while X(Fe) and X(Mn) of whole rock a re 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 (SiO 2 congruent to 74 %), can be interpreted as a consequence of the incom patible behaviour of this element in highly evolved granites, that are sufficiently poor in Ti, Mg and Fe to prevent or diminish crystalliza tion of ore-minerals, such as ilmenite.