CALCULATION OF THERMAL PEAK P-T CONDITIONS FROM SAPPHIRINE-BEARING GRANULITES

Sapphirine-quartz is a characteristic mineral assemblage of aluminous granulites formed at ultra-high temperature metamorphic conditions (T >950°C). Minimum P-T conditions for such granulites can be estimated using a petrogenetic grid for an appropriate model system, but accurate calculation of the thermal peak P-T conditions is important to quantify exactly how hot the continental crust can become during orogenesis. Two main problems are associated with calculation of thermal peak P-T conditions for such granulites: i) lack of precise thermodynamic data for sapphirine; ii) enhanced rate of Fe-Mg diffusion and consequent 'resetting' of mineral compositions during the post thermal peak evolution. Thus, it is necessary to use a recalculation method to retrieve peak mineral compositions [e.g. we used the protocol of Pattison & Bégin (1994, JMG, 12, 387-410)], before using e.g. garnet-orthopyroxene thermobarometry. Coexistence of these mineral assemblages with melt is to be expected, which creates one further problem, that of Ca depletion in garnet, which affects determination of P at maximum T. We exemplify these issues using granulites from the Anápolis-Itauçu Complex, Brazil, where sapphirine-bearing (± Qtz) granulites are found at several localities. Orthopyroxene is present in all samples examined; Opx has high Al2O3, with maxima of 12.9 wt% (ML-67, Spr-Qtz), 11.9 wt% (PT-62-A, Spr-Qtz), 10.9 wt% (PT-62-F, no Qtz) and 9.7 wt% (ANA-287, Spr inferred). Using recalculated mineral compositions for garnet-orthopyroxene pairs, T-P for these samples (°C/kbar) is 1012/9.4, 960/8.1, 975/8.5 and 975/9.7. In contrast, estimation of T graphically, using a petrogenetic grid for the FMAS system and isopleths of Al content in Opx, yields T >1150°C for ML-67 and PT-62-A and T ~ 1100°C for ANA-287. Graphical estimation yields higher peak temperatures that we consider to be more accurate than those calculated based on recalculated mineral compositions, at least for rocks of this study. In PT-62-A, a Ca depletion halo in garnet is common around multiphase (quartz-plagioclase) inclusions that we infer to have been melt pockets. Thus, we infer that interaction with melt was responsible for observed depletion. Low Ca contents leads to lower calculated pressures, which affects calculation of thermal peak temperatures.