Partial melting conditions in migmatitic gneisses from the Campo Maior Unit (Arronches, Portugal)

Abstract

The Campo Maior Unit (Variscan suture in the Ossa-Morena Complex) is formed by gneisses (paragneisses ± orthogneisses) ± amphibolites, with different degrees of migmatization. The gneisses vary from zones without migmatization to metatexites and diatexites, with partial-melting ages dated at 335–345 Ma. In a representative sample (N de Arronches) these gneisses present diatexitic textures with a mineralogy formed by Qz, Pl, Bt, Grt, ± Kfs, ± Sill, and Ilm (usually oriented and defining the main foliation). Bt, Grt and feldspars exhibit reaction textures. Sericite and chlorite are secondary phases. The whole-rock geochemistry is peraluminous (A/CNK = 1.91), with high TiO2 content (0.87 wt%) and relatively low CaO (1 wt%) and P2O5 (0.13 wt%) for a concentration of SiO2 of 64.5 wt%. To establish P, T and H2O conditions of this crustal melting event, P–T and T–XH2O pseudosections were calculated with Thermocalc in the MnNCKFMASHTO system, together with the ds62 thermodynamic database and its corresponding models of activity-composition for minerals that form solid solutions. By calculating isopleths of melt, Grt and Kfs, we established a P–T region where the calculated mineralogy coincides with the observed one (Qz + Pl + Kfs + Bt + Grt + Sill + Ilm) at 732–760ºC and 6.3–7.7 kbar, coexisting with 20–25% granitic melt. If we decrease the amount of H2O in the system towards the minimum necessary for saturation at these pressures (4.6–5 mol%), the melt is reduced by ∼ 10–15%, which could explain the different degrees of migmatization of these gneisses.