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 (peralum inous S-type granites, PS). The second is composed of amphibolejallanite-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; cor dierite 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. IV11AI diagram biotites are located in different fi elds according to the peraluminosity degree of the granite type. The estimated Fe3+j(Fe3+ +Fe 2 +) ratio of biotite is higher in PI than in PS granites, although this value is very pluton-dependent. Several compositional variations of AI-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 XMn, can be used as fractionation indices. Thus, the more acid the magma, the higher XFe and XMn in ATFM minerals. This correlation is not linear showing a rapid increase for silica values of Si02 � 74 %. Usually, XFe garnet> XFe biotite > XFe cordierite, and XMn ilmenite == XMn garnet> XMn cordierite > XMn biotite, while XFe and XMn 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 (Si02 == 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.

Multiple crustal sources for post-tectonic I-type granites in the Hercynian Iberian Belt

“…Acces sory almandine-spessartine garnet sometimes appears in leucogranite facies. More rare is the coeval presence of garnet with Fe-Mn-rich cordierite in some leucogranites (La Cabrera pluton, Villaseca and Barbero 1994). In the most felsic varieties of La Pedriza pluton, a highly fractionated rare-metal-rich leucogranite, a complex accessory paragen- …”

Andalusite and Na- and Li-rich cordierite in the La Costa pluton, Sierras Pampeanas, Argentina: textural and chemical evidence for a magmatic origin

“…The Al 2 0 3 content (31.64-32.84 wt%) is high relative to other Na-rich cordierites (1.65 :::: Na20 :::: 1.99 wt% and 28.2:::: A1 2 0 3 :::: 29.5 wt%) from pegrnatites (Gordillo et al 1985), but is similar to those from felsic monzogranites (e.g. Villaseca and Barbero 1994;Erdrnann et al 2004). However, the Santa Cruz facies cordierite is remarkably rich in Na20 (1.55-1.77 wt%), which is unusual for cordierites from granitic rocks of relatively low silica content (i.e.…”

“…Muscovite "a" and "b, c" (subscripts) correspond to primary and secondary muscovite, respectively (see text). Abbreviations mineral after KIetz (1983 ) bdl below detection limit et al 199 1;Villaseca and Barbero 1994;Cerny et al 1997 ;Rapela et al 2002;Erdmann et al 2004;Dahlquist et al 2005). Trace elements provide complementary information on the origin of the minerals, particularly for the case of cormerite (e.g.…”

“…The occurrence of Na-rich cormerite has been reported in sorne felsic to highly felsic granitoids (Si0 2 >71 wt%, e.g. Erdrnann et al 2004;Villaseca and Barbero 1994). However, the presence of Na-rich cormerite in granitic rocks of relatively low silica contents such as the Santa Cruz igneous facies (Si0 2 = 65.4---68.6 wt%) has not been found in the literature so faro We provide evidence that supports a magrnatic origin for these minerals and evaluate the substitution mechanism that led to this distinctive cormerite composition.…”

Textural and compositional evidence for a polyphase saturation of tourmaline in granitic rocks from the Třebíč Pluton (Bohemian Massif)

“…The elevated phosphorus contents in feldspar (Fig. 8a) and X Fe in biotite (0.81-0.91) from pegmatite dykes higher than in biotite from the marginal part of MBG (X Fe = 0.69-0.75) suggest the higher degree of fractionation of the pegmatites with tourmaline (see also Villaseca and Barbero 1994).…”