Metamorphic evolution of the Cinco Cerros area, Buenos Aires Province, Argentina

MASSONNE, H-J; DRISTAS A. JORGE; MARTÍNEZ JUAN CRUZ

Mar del Plata

Congreso; 10th International Geological Congress on the Southern Hemisphere (GEOSUR); 2010

Istituto Nazionale di Oceanografia e di Geofisica Sperimentale

The metamorphic basement of the WNW-ESE-trending Tandilia range in the Buenos Aires Province represents the southernmost extension of the Rio de la Plata craton and consists mainly of meta-igneous rocks. Occasionally, garnet-bearing metapelites crop out which have a much higher potential for the derivation of metamorphic P-T conditions than the garnet-absent, meta-igneous rocks. For this reason, we have studied metapelites of the Cinco Cerros area in the northeastern Tandilia range. In this area various lithologies occur which partially show migmatitic structures. However, gneiss sample 06181109 does not show any evidence for partial melting. Garnet in this sample is slightly zoned with core and rim compositions of pyrop17gross6spess2alm75 and pyrop13gross5.5spess1.5alm80, respectively. The Mg to Mg+Fe ratio of biotite is around 0.49. The plagioclase composition is Ab65An34Kf1. No cordierite, staurolite, and Al2SiO5-phase were noted to occur. We used the PERPLE_X computer software package to calculate a P-T pseudosection for sample 06181109. The calculated pseudosection was contoured by isopleths of various parameters especially molar fractions of garnet components. According to the latter calculation results we derived P-T conditions of 6.5 kbar and 670°C for an early metamorphic stage. Subsequently, a pressure release occurred at decreasing temperatures. The final metamorphic P-T conditions recorded by the studied rock are 4.5 kbar and 600°C. The corresponding P-T path is compatible with the observed mineral assemblage and the fact that in rocks adjacent to that of sample 06181109 partial melting occurred. As our study area is close to the margin of the Rio de la Plata Craton, where abundant magmatic-arc derived plutonic rocks are outcropping, we interpret the derived P-T path, which is probably related to events of the Transamazonian cycle about 2.2 - 1.8 Ga ago, by a heating event (not recorded by the studied rock) due to underplating of magmas of the magmatic arc. This event was followed by slow exhumation, probably caused by superficial erosion, accompanied by thermal relaxation. On the basis of a study of metamorphic rocks from an area nearby our study area, Delpino and Dristas (2008) proposed thinning of the crust possibly by the formation of a marginal back-arc basin. However, these authors deduced a somewhat different P-T path starting at 750-800°C and 5-6 kbar and ending at 450-500°C and 5.5-6.5 kbar. This demonstrates the necessity to derive P-T paths as precise as possible in order to suggest the right geodynamic model. For this reason, we will continue our work in the Cinco Cerros area by studying the complete lithological variety outcropping there.