Coronas, reaction rims, symplectites and emplacement depth of the Rymmen Gabbro, Transscandinavian igneous belt, southern Sweden

Goteborg University, Department of Geology, Goteborg, Sweden

Reactions between olivine and plagioclase form kelyphytic corona textures in the Rymmen gabbro, southern Sweden. The coronas surround olivine and consist of enstatite+ or -amphibole and phlogopite, combined with an outer rim of symplectic intergrowth of green spinel and amphibole. Corona reactions took place in fractures within olivine prior to the formation of chlorite, serpentine, and magnetite in the fractures at cooler conditions. Disequilibrium between olivine and plagioclase has been put forward as an explanation for the corona formation. However, inclusions of plagioclase in olivine show that there is no reaction between these minerals, since no new minerals formed between them. The conclusion drawn is that the most important factor for development of kelyphytic structures in the Rymmen gabbro is contact with a late deuteric fluid in an environment where olivine and plagioclase are close to each other. Temperature estimates for the formation of the kelyphytic coronas in the Rymmen gabbro yield consistent temperatures of around 800+ or -30 degrees C and pressures 6-8 kbar, which indicate emplacement at a depth of 20-30 km in the crust. Late deuteric fluids caused olivine replacement by intergrown orthopyroxene and magnetite, in Fe-Ti oxide-rich rocks only. In these, plagioclase is replaced by sympletctite when in contact with ilmenite and magnetite. It is noted that the symplectite minerals have low Ti contents. This suggests that the Fe-Ti oxide was a catalyst for the reaction between plagioclase and late-stage aqueous fluids rather than being a reactant, alternatively that the magnetite/ilmenite was a reactant and Ti was retained in the oxide.

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