CONICET | Buscador de Institutos y Recursos Humanos

The Sierras Pampeanas near Córdoba consist mainly of medium degree metamorphic rocks (tonalitic gneisses, amphibolites and marbles) grading to migmatitic belts trending NNW represented by homogeneous and heterogeneos cordierite-bearing migmatites. The San Carlos Massif represent the biggest and it is situated in the north-western part of the Sierras. Is composed mainly by homogeneous tonalitic migmatites (plagioclase-quartz-biotite-garnet-cordierite ± potash feldspar). At Río Hondo, in the southeastern part of the San Carlos Massif, stromatic migmatite are intruded by homogeneous migmatites (diatexites). It is possible to identify septa of quartz-feldespathic gneisses, amphibolites and marbles. All those rocks are intruded by dykes, veins and pegmatites in a complex pattern. Veins with a well developed comb texture are mineralized with auriferous sulfides (pyrite). Based on textural paths followed by rocks croping out at Río Hondo a clockwise loop is established. After overcome stability of St + Qtz and Ms + Qtz conditions, these rocks reached peak conditions (M2) producing anatexis and developing the main S2 foliation. Diatexites suffered a partial remobilization and intruded the metatexites. After the metamorphic peak a decompressive event (M3) took place, producing cordierite and plagioclase coronas around garnet and potash feldspar in the matrix. A cooling event (M4) followed decompression is evidenced by biotite coronas around garnet. The path was closed during the M5 event developed at lower temperatures producing pinnite, chlorite, calcite and clinozoisite.Previous proposed textural equilibria of cordierite-garnet during peak conditions M2 was not able to be proved. Instead, it was allowed to check the consumption of the garnet by cordierite and proving that cordierite is not a prograde phase but a retrograde one developed during M3 through solid-solid reactions. Metamorphic and deformational events can be summarized in two stages. Stage One are dominated by metamorphism and ductile deformation, and Stage Two affects rigid lithologies deformed in a brittle manner without metamorphism but magmatism associated with gold-bearing veins emplacement. -Stage One: deformational event D2 concomitant with M2 peak conditions developed S2 foliation related to axial plane of recumbent folds (F1). Previous foliation F1 forming intrafoliar folded foliation S1 (F1) and relictic paragenesis St + Qtz y Ms + Qtz allow to deduce a M1-D1 event prior to M2-D2. During deformational event D3 intrafoliar extensional shear movements occurred producing strong asymmetric folds F2. This event D3 was related to mobilization and emplacement of diatexites. Both folds (F1 + F2) were reworked by macroscopically smooth coaxial F3 folding by the D4 deformational event with axis plunging to the E and S4 axial planes trending NE-SW and dipping to S. The D5 deformational event produced centimetric reverse mylonitic bands (S5) cutting at high angles the main S2 foliation. This event was associated with uplift and thrusting of cortical scale blocks that provoked unroofing and decompression evidenced by retrograde coronas during M3 and M4 metamorphic events. The general orientation nearly E-W of S2 foliation and the F3 folding is anomalously oriented considering standard trends NNW with E dipping of S2 produced by folds verging to the west and subhorizontal axis NNW-SSE at Sierra de Córdoba.-Stage Two: during this stage extensional auriferous veins oriented E-W and dipping to the S were emplaced cutting S2 and the regional faulting. We propose here that these mineralizations were unrelated to the main metamorphism but it was linked to a regional brittle tectonic postdating lower Carboniferous emplacement of Achala batholit.

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