Coronas, reaction rims, symplectites and emplacement depth of the Rymmen gabbro, Transscandinavian Igneous Belt, southern Sweden

Abstract

Reactions between olivine and plagioclase form kelyphytic corona textures in the Rymmen gabbro, southern Sweden. The coronas surround olivine and consist of enstatite ± 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 ± 30°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 symplectite 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.

Corona structure from arc mafic-ultramafic cumulates: The role and chemical characteristics of late-magmatic hydrous liquids

“…The development of corona structures is commonly attributed to the chemical disequilibrium between reactants (Claeson, 1998). The reactants are either solids (mineral -mineral reaction) or solids and liquids (solid -liquid reaction).…”

Origin of the pleonaste-bearing mafic-ultramafic rocks from the Armutlu peninsula, NW Turkey

“…For example, pleonaste is extensively observed in the reaction texture between olivine and plagioclase in olivine-gabbro and troctolite (e.g., Claeson, 1998;Jan et al, 1992;Berger et al, 2010;Urraza et al, 2015;Khanchuk and Vysotskiy, 2016). This reaction is expressed by: (i) plagioclase + olivine ± H 2 O = clinopyroxene + orthopyroxene + spinel ± amphibole.…”

“…The Al-rich spinel solid solution series have been observed in the amphibolite-granulite facies basic-ultrabasic and pelitic rocks, the basic-ultrabasic magmatic rocks, and the mantle xenoliths in volcanic rocks (e.g., Evans and Frost, 1975;Babu et al, 1997;Ho et al, 2000;Bucher and Frey, 2002;Topuz et al, 2004;Amortegui et al, 2011;Daczko et al, 2012;Rodriguez et al, 2012;Gargiulo et al, 2013). Diverse origins, such as magmatic, metamorphic, and metasomatic, have been proposed for the Al-rich spinel occurrences in these rocks (e.g., Evans and Frost, 1975;Della-Pasqua et al, 1995;Claeson, 1998;Franz and Wirth, 2000;Berger et al, 2010;Daczko et al, 2012;Gargiulo et al, 2013).…”

Constraints on the origin of gabbroic rocks from the Moldanubian-Moravian units boundary (Bohemian Massif, Czech Republic and Austria)

“…de Haas 2002), but olivine-plagioclase coronas were described from low-pressure (up to 1 kbar) domains (Turner & Stuewe 1992). They may be formed either by late magmatic processes during reaction of cumulus phases with hydrous interstitial liquids, or in the solid state due to the activities of fluid-assisted metamorphic processes (Claeson 1998;de Haas et al 2002;Ikeda et al 2007;Helmy et al 2008).…”