Reaction rims in gabbros and their petrological significance, Valle Fértil and La Huerta Ranges, San Juan Province, Argentina.

MOGESSIE, ABERRA; SCHNEIDER, ISABELLA; GALLIEN, FLORIAN; CASTRO DE MACHUCA, BRÍGIDA; BJERG, ERNESTO; DELPINO, SERGIO; PREVILEY, LORENA; PONTORIERO, SANDRA; MEISSL, ESTELA; KOSTADINOFF, JOSÉ

Meran, Southern Tyrol

Workshop; MITT. ÖSTERR. MINER. GES. 153 (MinPet 2007); 2007

Österreichischen Mineralogischen Gesellschaft

The Sierras de La Huerta and Valle Fértil belong to the Western Sierras Pampeanas located in San Juan province, NW Argentina. This range consists of metagabbro to metadiorite as well as associated tonalite-granodiorite to granite that intruded the proterozoic basement in the Early-Middle Ordovician (Famatinian Orogeny). Gneisses, migmatites, amphibolites, marbles and calcsilicates represent the basement rocks. The abundant gneissic rocks which are strongly deformed and metamorphosed at granulite facies conditions consist mainly of Qtz-Pl-Bt-Grt-Sil-Crd±Kfs±Sp±Mag. The gabbros are composed of Pl(An70-100>)+Opx(En85-60Fs15-40Wo0-1)+Cpx(Wo50-40En45-50Fs5-10)+Ol(Fo58-70)+Spl+Ilm+Mag. Different textural occurrences of spinel (an Al-rich hercynite) have been documented: i) in the matrix, ii) in the symplectites with amphibole and iii) as exsolution lamellae in the Cpx. Amphibole is mostly pargasitic. Coronitic textures are well developed at Ol-Pl grain boundaries, and consist of Ol-Opx-Cpx-Am±Spl-Pl suggesting a reaction Ol+Pl = Opx+Cpx+Spl. Where H2O is involved clinopyroxene is replaced by amphibole. In some cases the reaction producing amphibole is: Ol+Pl+H2O = Opx+Amp+Spl. Most of the investigated coronitic gabbro samples contain spinel-pyroxene and spinel-amphibole symplectites which may indicate an open system where diffusion and transport of elements have been possible. These corona textures or reaction rims in the gabbros preserve evidences of changes in the environmental conditions during its history. As such, they are here interpreted to be the result of slow cooling of the mafic-ultramafic bodies under high amphibolite to lower granulite metamorphic conditions. Metamorphic peak coincided with corona formation and was related to the Famatinian intrusive activity. Geothermometric calculations based on the mineral parageneses (Grt-Bt, Grt-Crd, Gt-Sil-Qtz-Pl) in the associated high-grade gneisses as well as in the coronitic gabbros (Cpx-Opx) give pressure of 7.5-8.5 kb and temperature of 750-850 °C.