Peraluminous sapphirine in retrogressed kyanite-bearing eclogites from the South Carpathians: Status and implications

A retrogressed eclogite from the Lotru Metamorphic Suite in the South Carpathians containing peraluminous sapphirine in the fine-grained breakdown assemblage replacing kyanite was subjected to detailed petrographic and analytical studies. Textural analysis reveals that sapphirine is not directly related to decomposing kyanite; instead it grew by digestion of former spinel-bearing symplectites, which, together with corundum + plagioclase intergrowths, replaced kyanite as a first step. Initial kyanite breakdown to plagioclase, spinel, and corundum occurred through metasomatic addition of Ca, Na, Mg, and Fe, while Al and Si behaved as immobile components at the scale of the aggregate. The mechanism responsible for the breakdown was fluid-mediated intergranular diffusion of the mobile elements. All kyanite pseudomorphs exhibit a domain structure containing the alternative corundum + plagioclase or spinel + plagioclase two-phase assemblages, bounded by monomineralic plagioclase seams, across which short-scale Si transfer is apparent. The replacement of kyanite took place with conspicuous volume expansion. The amount of volume increase, as well as the modal proportions of the product phases, directly reflect the activity ratios of the mobile components. The sapphirine overprint is confined to the spinel-bearing symplectite. Sapphirine laths display a preferred orientation, discordant to grain boundaries and the orientation of the digested intergrowth. We propose a nearly constant volume replacement of the former spinel-bearing symplectite that is ascribed to an increase of the Ca/Na activity ratio during replacement. This inference is supported by recorded volume ratios of the phases involved and changes in plagioclase compositions. The succession of the two stages of kyanite replacement is characterised by irreversible reactions in the absence of bulk equilibrium, being therefore unsuitable for geothermobarometry. Thus, the mere presence of sapphirine-unlike as is often claimed-may not necessarily record evidence of a thermal pulse during decompression; such scenarios should be proven independently by more definitive features, such as coronitic aggregates with stable high-temperature assemblages. In the rock analyzed, peak temperatures slightly exceeding 700degreesC could only be calculated from the primary eclogitic assemblage, whereas garnet displaying well-preserved, strong compositional zonation shows evidence for a late-stage temperature decrease rather than increase.