Petrología, geoquímica y metamorfismo del Complejo Achiras, sur de la sierra de Comechingones

OTAMENDI, J.E., FAGIANO, M.R., NULLO, F.E, Y PATIÑO DOUCE, A.

Revista de la Asociación Geológica Argentina

Asociación Geológica Argentina

Año: 1998 vol. 53 p. 27 - 40

0004-4822

The Achiras Complex consists of s heterogeneous suite of medium-grade metarmorphic rocks derived from sedimentary protoliths. The most common metasedimentary rocks are biotite schists with compositions that resemble those of calcium-poor greywackes. Minor amounts of marbles, calc-silicate and amphibolites appear as small boudinaged bodies. The mineral assemblage of the schists is: biotite + plagioclase + quartz ± garnet ± muscovite ± sillimanite ± chlorite. Marbles are composed mainly of a calcite-dolomite matrix and also include phlogopite and tremolite. Calc-silicate rocks contain the assemblage: plagioclase + clinopyroxene + garnet + sphene, and the amphibolites are predominantly composed of hornblende and plagioclase. The Achiras Complex schists show a chemical pattern enriched in Mg, P, HFS and REE, and depleted in Al, Ca, Na, Ba and Sr. This geochemical signature is common in siliciclastic sediments transported far away from their source area, and is also found in the nearby Sierra del Morro schists, suggesting a genetic linkage with the Achiras Complex. Garnet crystals preserve the record of a polymetamorphic history, revealed by chemical zonation and petrography. During a first prograde stage garnet has growth broadly with the metamorphic peak. Garnet was then partially resorbed by surface dissolution in a retrograde stage. Despite the complex zoning and growth history of the garnet, the low ratio garnet to biotite in the rock constitute an ideal opportunity to estimate peak metamorphic temperature conditions of 700 ± 50º C. The geobarometer garnet-aluminosilicate-silica-plagioclase (GASP) yields a metamorphic pressure of 5.7 ± 0.9 kbar, on the basis of arbitrary fixed activity of aluminosilicate and for the range of the temperatures obtained from garnet-biotite equilibrium.