Thermobarometry at 32°00'S in the Pampean Ranges near Córdoba, Argentina

MARTINO, ROBERTO DONATO; MUNN, BARBARA; KRAEMER, PABLO; ESCAYOLA, MÓNICA; GUERESCHI, ALINA BEATRIZ

Seattle (EE.UU.)

Congreso; Geological Society of America, Abstracts with Program; 1994

Geological Society of America

New thermobarometric calculations from an E-W transect across the Pampean Ranges near Córdoba at 32º00´S constrain the regional metamorphism of the upper amphibolite grade basement rocks to higher pressures and temperatures than previously reported.  The transect crosses two blocks bound by Tertiary faults.  The Sierra de Comechingones (western block) is composed of stromatic migmatites and diatexites intercalated with ultramafic and mafic rocks, grt gneisses and kinzigite to the west, while biotite gneisses predominate to the east. The Sierra Chica (eastern block) presents a similar symmetry with diatexites, bt-grt and grt-sil gneisses with minor ultramafic rocks and amphibolites to the west and biotite gneisses to the east.  The dominant pelitic assemblage, found in the western portion of each block, is grt-bt-qtz-pl-kfs±sil±crd. Grt-bt thermometry and GASP barometry were applied, supplemented by grt-crd thermometry and grt-pl-bt-qz barometry.  The garnets exhibit varying degrees of resorption with embayed edges and irregular crystal outlines.  Partial retrograde resorption is corroborated by garnet profiles and X-ray images which show strong Mn- and Fe-enrichment at the rims.  Grt points inside of the modified rim were chosen for thermobarometry to avoid the retrograde effects and are believed to be the closest possible approximation of peak rim conditions.  These grt compositions were paired with matrix bt and/or pl for the P-T calculations. The thermobarometric results are remarkably consistent across the transect with an average of 800°C and 8Kbar and are compatible with the observed phase equilibria and the migmatized nature of the blocks.  Earlier interpretations of the Pampean Ranges suggested that the basement deepens from west to east.  However, our results indicate that the western portions of each block formed under similar P-T conditions and that there is no difference between the eastern and western sierras.  Combining our data with that of Gordillo (1984) to the north (≈700°C & 6kbar), we propose that  progressively deeper levels of the crust are exposed in a southwesterly direction parallel to the trend of the principal regional foliation.