A previously unrecognised major orogenic front in Argentina

FINCH, MELANIE; FUENTES, GABRIELA; HASALOVÁ, PAVLINA; BECCHIO, RAÚL; HUNTER, NICK; WEINBERG, ROBERTO

Brisbane

Congreso; 34th International Geological Congress; 2012

34th International Geological Congress

Ultramylonites are most commonly found in greenschist facies rocks where they appear as cm-thick bands contained within rocks of lower strain. This poster documents a kilometre-thick, high-temperature ultramylonite zone in the Sierra de Quilmes, part of the Sierras Pampeanas of Argentina. The Sierra de Quilmes is a 140 km long metamorphic complex that underwent deformation and anatexis at 470 Ma, during the Famatinian orogeny. The complex was thrust to the SW and has a high strain zone of at least 5 km comprising mylonites and ultramylonites, designated here the El Pichao Shear Zone. Within this high strain zone, there is a one kilometre thick core of continuous ultramylonite, overprinting migmatites, granites and a sedimentary sequence. The only other ultramylonite of a similar thickness described in the literature (Striding-Athabasca mylonite zone, Hanmer et al., 1995) has been attributed to mass loss and high temperature recrystallisation. However, protomylonites and ultramylonites of the El Pichao Shear Zone are geochemically similar, so the ultramylonites are more likely to be a result of high strain distributed over a wide area indicative of poorly developed localisation processes. A thrust zone this thick and with such high strain most likely represents a major orogenic front during the Famatinian orogeny. The El Pichao Shear Zone consists of at least 1 km of continuous ultramylonite overprinting migmatites, granites and metasedimentary rocks. Shear zones of this thickness have been previously reported to be a result of high temperature recrystallisation and mass loss. However, REE patterns in the El Pichao mylonites indicate that proto- and ultramylonites are not chemically different and therefore the ultramylonites are not a result of mass loss. In the granitic mylonites feldspar porphyroclasts show three main behaviours: (a) brittle fracture, (b) recrystallisation, or (c) rotation. Quartz ribbons wrap around feldspar porphyroclasts. When these porphyroclasts recrystallise and shear they mix with the wrapped quartz and form a homogenous matrix. This decreases the connectivity of the phases. We are investigating the origin of such unusually thick ultramylonites and what inhibits the localisation of strain. It is our conclusion that a thrust zone this thick and with such high strain must be a major orogenic front of the Famatinian orogeny.