RT Journal Article
T1 Chemical variability of Al-Ti-Fe-Mg mineralsin peraluminous granitoid rocks from Central Spain
A1 Villaseca González, Carlos
A1 Barbero González, Luis C.
AB 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 (peraluminousS-type granites, PS). The second is composed of amphibole/allanite-bearing granites, weakly peraluminousin composition (peraluminous I-type, PI). These granite types evolved through complex igneous fractionationprocesses.Both peraluminous granite types show differences in the chemical composition of biotite and cordierite; cordieriteis common in PS granites but restricted to some highly fractionated PI granites. Garnet and ilmenite cannotbe 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 Nacontent in biotite (and cordierite) of PS granites is noteworthy. When plotted in a Na vs. [VI1A1 diagram biotitesare located in different fields according to the peraluminosity degree of the granite type. The estimatedFe3+/(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 fractionationor evolution of the magma, irrespectively of its peraluminosity. Other chemical features of ATFM phases, suchas XFe or XMΠ, can be used as fractionation indices. Thus, the more acid the magma, the higher XFe and XMΠ inATFM 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, whileXFe and XMΠ of whole rock are very close to biotite values. The high increase in Mn content, more marked thanin Fe, in all AFM phases in the silica-rich granites (SiO2 = 74 %), can be interpreted as a consequence of theincompatible behaviour of this element in highly evolved granites, that are sufficiently poor in Ti, Mg and Fe toprevent or diminish crystallization of ore-minerals, such as ilmenite.
PB E. Schweizerbart'sche Verlagsbuchhandlung
SN 0935-1221
YR 1994
FD 1994
LK https://hdl.handle.net/20.500.14352/57028
UL https://hdl.handle.net/20.500.14352/57028
LA eng
DS Docta Complutense
RD 4 abr 2025