Abstract

CRISTOFOLINI, Eber et al. Geology of  the western portion of  the Sierra de Valle Fértil, San Juan, from observations in the Quebrada de Otarola. Rev. Asoc. Geol. Argent. [online]. 2010, vol.67, n.4, pp.521-535. ISSN 0004-4822.

This work presents field relationships, petrography and structural features of  the crystalline complex that form the central-western portion of  the Valle Fértil range, San Juan. The crystalline complex comprises five main lithologic associations: gabbroic rocks, dioritic rocks, metasedimentary migmatites, pegmatitic granites and mylonite series rocks. The fabric igneous preserved in the gabbroic association is which manifest as either layering of  cumulate origin (S_0a) and magmatic flow bands (S_0b). In most places, the primary fabrics occur reworked and/or cross-cut by the submagmatic foliation (S₁). However, S₁ is associated with low differential stress and the only changes in the primary igneous and metamorphic mineral assemblages are growth of  hornblende in mafic rocks and biotite + sillimanite in the metasedimentary rocks. This suggests that (D₁) deformation acted under high temperature and synchronously with the igneous activity. Subsequent deformation (D₂) is related to high-strain shearing and formed the fabrics S_2a and S_2b associated a typical mylonite series rocks. Kinematic analysis in mylonites indicates top-to-west reverse sense of  shear during D ₂ deformation. All structures are locally truncated by brittle planar features (S₃). The u-Pb zircon age of  474 ± 4.7 Ma for a metasedimentary migmatite dates the time of  the metamorphic peak. This also proves that the metasedimentary rocks reached granulite-facies conditions at the same time that igneous activity was active.

Keywords : Famatinian orogen; Valle Fértil range; Gabbro; Shear zone; Structural evolution.

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