Per aluminous granites: The effect of alumina on melt composition and coexisting minerals

Liquidus phase relationships at H₂O-saturated and -undersaturated conditions and 2 kbar in the systems Qz-Or-Ab (SiO ₂-KAlSi₃O₈-NaAlSi₃O₈), Qz-Or-Ab-Al₂O₃, and subsystems are compared and discussed. In the peraluminous systems (i.e. when melts are saturated with respect to mullite) the liquidus temperatures are lowered by 40-55°C for compositions in the quartz primary field and by 15-25°C for cotectic compositions. The composition of the Qz-Ab eutectic and of the minimum are slightly shifted towards more Qz-rich compositions (minimum composition at P(H₂O) = 2 kbar in the system Qz-Or-Ab-A1₂O₃ saturated with respect to mullite: Qz₄₀Or₂₃Ab₃₇). In melts saturated with sillimanite or mullite, the effect of high A1 content may be lower for the Qz-Or than for the Qz-Ab eutectic. The depression of the liquidus temperatures may be partly related to the higher H₂O solubility in melts saturated with respect to mullite. The solubility of H₂O in a melt with a composition of Qz₂₈Or₃₄Ab₃₈ at 2 kbar and 800°C is 5-77 ± 0-15 wt% H₂O and 6-36 ± 0-30 wt% H₂O in a melt with the same Qz/Or/Ab proportions but saturated with respect to mullite. The effect of high A1 contents on the Mg and Fe contents of Ca-free granite melts was investigated at 775°C-3 kbar (melts coexisting with phlogopite), and at 820°C-2 kbar (melts coexisting with biotite and spinel), under NNO buffer conditions. Less than 0-15 wt% MgO is incorporated in subaluminous melts coexisting with phlogopite, whereas peraluminous melts (2-9 wt% normative corundum) contain 0-6-0-7 wt% MgO. A similar behaviour of the MgO content is observed for melts coexisting with biotite. In contrast, no significant effect of high A1 contents on the FeO content of melt coexisting with biotite was observed. This suggests that the Fe/Mg ratio may be significantly lower in peraluminous than in subaluminous granitic melts.