Mineralogical and microstructural characterization, and post-depositional history of La Aguadita Formation, Famatina System,La Rioja, Argentina

Abstract

The La Aguadita Formation crops out in the core of the Famatina Range (28º39’ S) in the southern Central Andes and is interpreted as a relict of a synorogenic Ordovician clastic wedge developed in the retroarc of a collisional-subduction margin built along western Gondwana. This unit, formerly included within the Pampean (Early Cambrian) metamorphic complex, was recently attributed to the middle Ordovician on the basis of U-Pb SHRIMP dating on detrital zircons. La Aguadita Formation comprises two informal members, the lower one is composed of metarkoses with clastic to blastopsammitic textures, whereas the upper member consists of metapelites with subordinated thin-bedded metagraywackes. Mineralogical (XRD) and petrological analyses carried out for the rocks outcropping in Loma de Las Damas indicate that La Aguadita Formation underwent an incipient regional metamorphic event, in which the mineral blastesis was synchronous with the development of the S1 cleavage. According to the XRD study of the <2 µm fraction, the main mineral formed during this event was illite, with subordinate chlorite. Besides, the petrographic study evidences recrystallization processes in quartz and feldespate. The values obtained for the Kübler index of the metapelites correspond entirely to the anchizone (IK= 0.25–0.40 Δº2θ), indicating that the main tectono-metamorphic event took place at temperatures around 200–300 ºC and depths of 6.5 to 10 km. The existence of blastic epidote and chlorite in some metarkoses and metagraywackes, together with their textural relationships denote a propilitic hydrothermal alteration episode that postdates the main tectono-thermal episode. The clay fraction of the metapelites contains–in addition to the main phyllosilicates– smectite, berthierine, vermiculite, and I/S and Chl/Vm mixed- layered minerals. This group of phyllosilicates is incompatible not only with the grade established for the main metamorphic event, but also with the temperatures estimated for the epidote + chlorite assemblage developed during the hydrothermal alteration. They are interpreted as the result of a retrograde diagenesis episode promted by low-temperature, late-stage hydrothermal fluids or by the interaction between rocks and groundwater after uplift.