Modelling of mineral equilibria in ultrahigh‐temperature metamorphic rocks from the Anápolis–Itauçu Complex, central Brazil

Abstract

A new quantitative approach to constraining mineral equilibria in sapphirine-bearing ultrahightemperature (UHT) granulites through the use of pseudosections and compatibility diagrams is presented, using a recently published thermodynamic model for sapphirine. The approach is illustrated with an example from an UHT locality in the Ana´polis-Itauc¸u Complex, central Brazil, where modelling of mineral equilibria indicates peak metamorphic conditions of about 9 kbar and 1000°C. The early formed, coarse-grained assemblage is garnet-orthopyroxene-sillimanite-quartz, which was subsequently modified following peak conditions. The retrograde pressure-temperature (P-T) path of this locality involves decompression across the FeO-MgO-Al 2 O 3 -SiO 2 (FMAS) univariant reaction orthopyroxene + sillimanite ¼ garnet + sapphirine + quartz, resulting in the growth of sapphirine-quartz, followed by cooling and recrossing of this reaction. The resulting microstructures are modelled using compatibility diagrams, and pseudosections calculated for specific grain boundaries considered as chemical domains. The sequence of microstructures preserved in the rocks constrains a two-stage isothermal decompression-isobaric cooling path. The stability of cordierite along the retrograde path is examined using a domainal approach and pseudosections for orthopyroxene-quartz and garnet-quartz grain boundaries. This analysis indicates that the presence or absence of cordierite may be explained by local variation in a H 2 O . This study has important implications for thermobarometric studies of UHT granulites, mainly through showing that traditional FMAS petrogenetic grids based on experiments alone may overestimate P-T conditions. Such grids are effectively constant a H 2 O sections in FMAS-H 2 O (FMASH), for which the corresponding a H 2 O is commonly higher than that experienced by UHT granulites. A corollary of this dependence of mineral equilibria on a H 2 O is that local variations in a H 2 O may explain the formation of cordierite without significant changes in P-T conditions, particularly without marked decompression.

Wollastonite–scapolite–clinopyroxene marble of the Anápolis-Itauçu Complex, Goiás: more evidence of ultrahigh-temperature metamorphism

“…Crossing reactions 2 and 4 implies decreasing X CO2 with almost no cooling, and crossing reactions 2, 3, and 5 implies cooling with a small decrease in X CO2 . We prefer the second interpretation, because it implies a similar P-T path to those inferred for other portions of the Anápolis-Itauçu Complex (Moraes et al 2002, Baldwin et al 2005. A value of X CO2 between 0.1 and 0.3 is inferred to be consistent with these reaction microstructures (Fig.…”

“…Such extreme granulites were previously described in Brazil in rocks from the Itabuna-Salvador-Curaçá Belt in Bahia (Stormer Jr. 1973, Leite 2002, and the Niquelândia Complex (Ferreira Filho et al 1998), Barro Alto Complex (Moraes & Fuck 2000) and Anápolis-Itauçu Complex (Moraes et al 2002, Baldwin et al 2005 in Goiás. In Brazil, UHT granulites occur as isolated outcrops within 'common' granulites, in similar fashion to those described from Antarctica (Harley & Hensen 1990) and India (Brown & Raith 1996).…”

“…North of Goiânia (ML-67), an impure quartzite preserves aluminous orhopyroxene+sillimanite+quartz and sapphirine+quartz assemblages; reaction microstructures and high Al 2 O 3 in orthopyroxene (12.9 wt. %) allowed the inference of a composite P-T path, with discrete decompression followed by a near-isobaric cooling stage from T >1,000°C at >9 kbar to <900°C (Moraes et al 2002, Baldwin et al 2005. Near Damolândia (PT-62) and in the Monjolo stream (ANA-287), sapphirine occurs in both quartz-rich and quartz-poor rocks.…”

Applications and limitations of thermobarometry in migmatites and granulites using as an example rocks of the Araçuaí Orogen in southern Bahia, including a discussion on the tectonic meaning of the current results

“…Here, the thermobarometric calculations are based on a group of linearly independent reactions that use all thermodynamic information of metamorphic peak mineral assemblages; consequently, the results produced are robust (Powell & Holland 1994). If it is possible to have the bulk composition of the protolith before granulite facies metamorphism and partial melting took place, for example, by analyzing an amphibolite facies regional equivalent, it is possible to calculate pseudosections and, through them, to make a model of metamorphism and partial melting processes (White & Powell 2002;Powell et al 2005;Baldwin et al 2005). The beauty of pseudosections is also its doom, because its calculation is based on the rock bulk composition, but in the case of migmatites and granulites, which involved partial melting and segregation of its melt phase, original rock bulk composition is lost, making it difficult or impossible to use the method for a precise thermobarometry.…”

Introduction to a virtual special issue on crustal melting

“…Microstructures such as corona and symplectites are commonly used to infer retrograde reactions, which are then used to track a path for a particular sample through a generalized P-T grid or a sample-specific P-T pseudosection constructed for suprasolidus conditions, although care must be taken to ensure the correct interpretation (e.g. Baldwin et al, 2005;White & Powell, 2011). Retrograde processes have important implications for how ages determined from accessory phases in high-grade metamorphic rocks should be interpreted.…”