Resumen

MOREIRA, P. et al. The metamorphic basement of very low to low grade of Estancias La Modesta and La Josefina, Deseado Massif, province of Santa Cruz. Rev. Asoc. Geol. Argent. [online]. 2005, vol.60, n.1, pp.049-063. ISSN 0004-4822.

The detail map of the type locality of the metamorphic rocks of La Modesta Formation is presented in this paper. The petrographic, geochemical and structural studies of these rocks let us to identify new lithologies of this unit (calcosilicate rocks, basic metavolcanics, tourmalinites, and Fe-Mn bearing beds). These studies also establish a very low to low metamorphic grade from prehnita-pumpellita to green schists facies. A S₁ schistosity which is subparallel to a S₀ sedimentary beds was founded. The age inferred from a geochronological analysis (errorchrone Rb-Sr: 413 ± 17 Ma), has been reinterpreted. This research allowed to correlate this formation with other sedimentary or low grade metamorphic units close to other regions of the Deseado Massif and to differentiate them from the Río Deseado Complex rocks. Finally, a possible pre-Mesozoic geologic evolution for this area related to a proto-Pacific margin is proposed.

Palabras clave : Low grade metamorphism; La Modesta Formation; Deseado Massif; Turmalinite.

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