On the origin of multi-layer coronas between olivine and plagioclase at the gabro-granulite transition, Valle Fertil-La Huerta Ranges, San Juan Province, Argentina

GALLIEN, FLORIAN; MOGESSIE, ABERRA; HAUZENBERGER, C.; BJERG, ERNESTO; DELPINO, SERGIO; CASTRO DE MACHUCA, BRIGIDA

JOURNAL OF METAMORPHIC GEOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2012 vol. 30 p. 281 - 302

0263-4929

Troctolitic gabbros from Valle Fertil and La Huerta Ranges, San Juan Province, NW Argentina, exhibit multi-layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence varies in composition from 0.5 - 1 mm thick multi-layer coronas comprising olivine | orthopyroxene | clinopyroxene+spinel symplectite | plagioclase and olivine | orthopyroxene | amphibole | amphibole+spinel symplectite | plagioclase. In our contribution we want to stress, that the coronas have formed not exclusively due to metamorphic solid state replacement of magmatic olivine and plagioclase. In some cases, the influence of an interstitial, stationary liquid, trapped between cumulus olivine and plagioclase reacts with olivine in a first instance to produce a rim of peritectic orthopyroxene around olivine. Subsequently, a first generation of inclusion free, brownish amphibole (I) evolves from the intercumulus liquid between peritectic orthopyroxene and plagioclase. Amphibole (I) is enriched in trace elements and REEs relative to a second generation of greenish amphibole (II). Amphibole (II) forms by metamorphic solid state replacement of peritectic orthopyroxene and plagioclase and is part of the outer layer sequence comprising amphibole amphibole+spinel symplectite. Diffusion controlled metamorphic solid-state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase but also orthopyroxene and plagioclase. Thus the thermodynamic incompatibility of the reactant minerals at the gabbro-granulite transition and the phase equilibria of the coronitic assemblage at subsequent cooling were modelled in the µMgO-µCaO space. It is demonstrated that the anhydrous corona assemblage comprising orthopyroxene | clinopyroxene+spinel symplectite and the hydrous corona assemblage of amphibole | amphibole+spinel symplectite have formed due to solid state replacement of pre-existing minerals at temperatures from 850 °C - <800 °C. Micro texture of the layer boundaries suggests a growth of orthopyroxene at the expense of olivine, while the clinopyroxene+spinel symplectite migrates outward at the expense of plagioclase. The position of the original olivine-plagioclase interface is located at the outer orthopyroxene boundary, adjacent to the clinopyroxene+spinel symplectite. At T >800 °C, clinopyroxene gets replaced by amphibole leaving a coarse grained amphibole+spinel symplectite behind. If clinopyroxene is not present, plagioclase gets replaced by an outward migrating amphibole+spinel symplectite. Calculation of the possible net-reactions by considering NCKFMASH components indicates that the layer bulk can not be modelled in an entirely "closed" system but in all cases the gain and loss of elements within the multi-layer coronas (except H2O, Na2O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and/or replacing olivine. The corona reaction textures indicate a counter clockwise metamorphic evolution at the gabbro-granulite transition at 5-6.5 kbar and temperatures below 900 °C.