Coronitic metagabbros and associated basement rocks of the Valle Fertil-La Huerta Ranges, San Juan Province, NW Argentina

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

Innsbrück

Conferencia; PANGEO Austria 2006; 2006

University of Innsbrück (Austria)

The two pyroxene metagabbros from the La Huerta-Valle Fertil Ranges show well preserved igneous textures and layering. They are composed of Pl (An70-100) + Opx (En85-60Fs15-40Wo0-1) + Cpx (Wo50-40En45-50Fs5-10) + Ol (Fo58-70) + Spl + Ilm + Mag. The spinel (an Al-rich hercynite) is found in the matrix, in the symplectites with amphibole and as exsolution lamellae in the Cpx. The chromium content is very low, and reaches 10 wt % Cr2O3 in the spinel exsolutions in Cpx. The amphiboles are mostly pargasitic hornblendes [K0,07Na0,5Ca1,9Mg3,0Fe2+0,9Fe3+0,12Al2,5Ti0,09Si6,2O22(OH,F,Cl)2]. The presence of large amount of symplectites (spinel-amphibole, plagioclase-amphibole) in most of the investigated coronitic metagabbros samples may indicate an open system where diffusion and transport of elements has been possible. Coronitic textures, which developed at Ol- Pl grain boundaries, are very common. They are composed of Ol + Opx + Cpx + Am + Àl Spl + Pl suggesting a reaction Ol + Pl = Opx + Cpx + Amph + Spl. They are interpreted to be the result of slow cooling of the mafic - ultramafic bodies under high amphibolite to lower granulite metamorphic conditions. Peak of metamorphism coincided with corona formation and was related to the Famatinian intrusive activity. Later, the metagabbros were locally cut by narrow ductile shear zones associated with mylonitization. Preliminary geothermometric calculations based on the mineral parageneses (Grt/Bt, Grt/Crd, Gt-Sil-Qtz-Pl) in the high-grade gneisses as well as in the coronitic metagabbros (Cpx/Opx ) result in pressure of 7.5- 8.5 kb and temperature of 750ºC-850 ºC.