Abstract
Phase equilibria modelling incorporating melt reintegration offers a methodology to create hypothetical rock compositions that may have existed prior to melt loss, allowing the potential prograde evolution of rocks to be explored. However, melt reintegration modelling relies on assumptions concerning the volume of melt that was lost and is generally restricted by the absence of direct constraints on the pre-anatectic mineral assemblages. Mg-rich granulite in the 514–490 Ma Delamerian Orogen in southern Australia contains spinel–cordierite symplectic intergrowths that surround rare, coarse blocky domains of sillimanite. These sillimanite cores, as well as the widespread presence of andalusite in lower grade areas of the southern Delamerian Orogen, suggest that the subsolidus precursor to the granulite contained andalusite. This provides the opportunity to test if melt reintegration modelling of the granulite predicts subsolidus andalusite. Stepwise down-temperature melt reintegration modelling produces a water-saturated solidus after the addition of 12 mol% melt. When modelled at subsolidus conditions, the resulting rock composition produces andalusite-bearing assemblages with andalusite modes similar to the abundance of the sillimanite-cored spinel–cordierite intergrowths. The modelling results from this case study suggest that melt reintegration modelling is a valid method to recreate prograde subsolidus bulk rock compositions.
Highlights
Melting and melt loss during granulite facies metamorphism is a fundamental process that differentiates the continental crust and causes chemical depletion in the residual material [1,2,3].Advances in thermodynamic modelling of mineral assemblages have enabled quantitative phase equilibria modelling of the melting processes operating in partially molten siliciclastic rocks in increasingly complex chemical systems [4,5,6,7,8,9,10,11]
We based interpret andalusite to have been present during the subsolidus prograde evolution of the rock, on andalusite to have been present during the subsolidus prograde of the rock, intergrowths, based on blocky blocky mineral aggregates cored by sillimanite that are rimmedevolution by cordierite–spinel as mineral aggregates cored by sillimanite that are rimmed by cordierite–spinel intergrowths, as well as well as the widespread presence of andalusite in lower grade stratigraphic equivalents
Phase equilibria modelling undertaken on the Kanmantoo sediments at Reedy Creek indicates that peak P–T conditions of 790–860 ◦ C and 1.0–4.4 kbar were reached
Summary
Advances in thermodynamic modelling of mineral assemblages have enabled quantitative phase equilibria modelling of the melting processes operating in partially molten siliciclastic rocks in increasingly complex chemical systems [4,5,6,7,8,9,10,11] These models have been used to investigate the effects of melting and melt loss on crustal rheology, composition and the preservation of geochronometers e.g., [1,5,12,13,14,15]. Quantitative petrological interpretation of mineral inclusions captured within prograde minerals
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