Abstract

TIBALDI, Alina M.  and  OTAMENDI, Juan E.. Mineralogy and thermo-barometry of the Sol de Mayo and Suya Taco mafic complexes from northern Sierra de Comechingones, Córdoba. Rev. Asoc. Geol. Argent. [online]. 2008, vol.63, n.1, pp.117-130. ISSN 0004-4822.

This work presents a detailed study including petrography, mineral chemistry and thermo-barometry of igneous rock from two mafic complexes that outcrop in the north portion of the sierra de Comechingones. The range of absolute P and T under which these rocks crystallized is retrieved using a set of empiric barometers and thermometers. The estimations of equilibrium temperature determined in pairs of pyroxenes (cpx-opx) were performed through three independent thermometers. Two of the thermometric formulations (Wells 1977, Kretz 1982) yield consistent and reasonable results indicating that these phases record magmatic crystallizations conditions (920-1150ºC), whereas the third calibration by Lindsley (1983) gives lower temperatures. This discrepancy is thought to reflect the fact that the latter method may be not applicable to rocks crystallized from Fe-rich magmas. Amphibole-plagioclase thermometry estimates, which use typical core compositions, yield equilibrium temperature in the range of 750-850ºC. Whereas using rim compositions, Amph-pl thermometry estimates significantly give lower temperatures (< 600ºC), suggesting that amphiboles mostly crystallized at post-magmatic stages. Orthopyroxene-biotite Fe-Mg exchange thermometry shows that these minerals tend to equilibrate within two distinct thermal ranges (430-650ºC and 740-1030ºC). This indicates that biotites might have crystallized at both late-magmatic and post-magmatic stages. Barometric estimates have some variability in function of the type of calibration used, however they show a systematic pattern in which the equilibrium pressure determined in the Suya Taco complex are lower (5.5 ± 1 kbar) than those obtained in the Sol de Mayo complex (8 ± 1 kbar). Overall, this result is consistent with barometry already performed in granulitic xenoliths. The P-T conditions deduced from igneous rocks indicate that the hosting regional metasedimentary sequence were experiencing amphibolite-facies temperatures and residing at middle crustal levels.

Keywords : Mafic rocks; Sierra de Comechingones; Thermometry; Barometry; Pampean orogen.

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