Cordieritite and leucogranite formation during emplacement of highly peraluminous magma: the El Pilón Granite Complex (Sierras Pampeanas, Argentina)

RAPELA, C.W.; BALDO, E.G; PANKHURST, R.J; SAAVEDRA, J.

JOURNAL OF PETROLOGY

OXFORD UNIV PRESS

Lugar: Oxford; Año: 2002 vol. 43 p. 1003 - 10028

0022-3530

Cordieritites and high peraluminous granites of the El Pilón granite complex were emplaced during the M3, low-P, high-T metamorphism, during initial decompression of the Cambrian orogen in the southwestern margin of Gondwana. Very fresh orbicular and massive cordieritite bodies with up to 90% cordieritite are genetically associated with a cordierite monzogranite pluton and a larger body of porphyritic granodiorite. The petrogenesis of this association has been studied using petrographical, mineralogical, thermobarometric, geochemical, geochronological and isotope methods. The granitic magmas were formed by anatexis of middle crustal metamorphic rocks formed earlier in the Pampean orogeny. The cordieritites appear at the top of feeder conduits that connected the source region located at about 6 Kbar with the pluton emplacement level at 4.0 ± 0.3 Kbar. A decrease in the liquidus temperature of the melt during emplacement was produced by a marked increase in fluid activity due to rapid decompression and injection of water from the environmental schists. The supercooling effect induced cordierite to crystallize on restitic solid objects and early formed cordierite crystals. Strong convection in the small magma chamber caused flow segregation of cumulate cordierite and restite, developing leucogranites and highly evolved pegmatoids that are in isotopic equilibrium. Rapid ascent of highly peraluminous magmas during the M3 metamorphism might explain why these granites were emplaced almost coevally with formation of the M2 garnet-cordierite migmatites at deeper levels in the crust.