C. Villaseca et L. Barbero, CHEMICAL VARIABILITY OF AL-TI-FE-MG MINERALS IN PERALUMINOUS GRANITOID ROCKS FROM CENTRAL SPAIN, European journal of mineralogy, 6(5), 1994, pp. 691-710
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.