The southern extension of the Sierras Pampeanas in La Pampa province: geological and geophysical evidences

Abstract

In Valle Daza, outcrops of granoblastites (derived from gabbros) and leucocratic granites with aplitic and pegmatitic varieties reveal the existence of two metamorphic events. Their characteristics are: a) ductile deformation in upper amphibolite facies or higher, responsible for the NNE foliation and the transformation of granoblastites into amphibolites, and of granites and pegmatites into protomylonites and mylonites, and b) low-temperature fragile-ductile deformation, leading to retrogradation of high-grade paragenesis into middle to upper greenschist facies. Significant gravimetric (diameter 30 km and +15 mGal amplitude) and magnetometric anomalies (width 10 km and 250 nT maximum value) in the study area, indicate the presence of important volumes of high density mafic rocks. The mafic and granitic rocks cropping out in the western part of Sierra de Lonco Vaca share very similar petrological and structural characteristics to those occurring in Valle Daza. The Lonco Vaca range is located at the northern prolongation of a series of gravimagnetometric anomalies bearing the same orientation as the regional NNE foliation, with remarkable local anomalies at the Estancia El Coya, Valle Daza, El Durazno and Victorica localities. From north to south, the outcrops of clinopyroxenic granoblastites and amphibolites at Lonco Vaca, Valle Daza and Estancia Santa María are coincident with gravimagnetometric anomalies. Our results indicate that the tectono-metamorphic events recognized in La Pampa province can be correlated with the F3 (S3-M3) and F4 (M4) events recognized in the central block of the Sierras de San Luis. Both regions share other common features, such as discontinuous NNE trending gravimagnetometric anomalies, coincident with regional belts of ductile deformation dipping ESE and with WNW vergence. Structural, petrological and geophysical correlations established in the present study, constitute a significant contribution in favour of the hypothesis postulating the extension of the eastern Sierras Pampeanas into the central portion of La Pampa province.