Resumen
The thermal post-depositional evolution of metapelitic and metavolcanic rocks of the upper CambrianOrdovician
succession in the Central Andes of northwestern Argentina, was estimated through X ray diffraction of clay
mineral analysis, Kübler Index (KI), SEM-EDS study of selected samples and chlorite geothermometry. The study area
comprise five representative regions in Puna and Cordillera Oriental, namely at Sierra de Santa Victoria (Nazareno
and Santa Victoria river areas), Cordón de los Siete Hermanos (Yavi), Sierra de Rinconada and Sierra de CochinocaEscaya.
The paleotemperatures obtained with the chlorite geothermometer, in coincidence with KI values, show an
E-W trend from diagenesis/low anchizone in the eastern flank of Sierra de Santa Victoria, to high anchizone/epizone in
the Puna, with intermediate values in the western flank of Sierra de Santa Victoria. This trend, in conjunction with the
development of slaty cleavage in the Ordovician rocks from Yavi, Sierra de Cochinoca-Escaya and western localities
coincide with the limits proposed for the ?Ocloyic deformation belt?. The common occurrence of kaolinite in the slates
and the metavolcanic rocks of Sierra de Cochinoca-Escaya, coupled with the substitution of chlorite by interstratified
lower-temperature phases in most of these rocks and the occurrence of jarosite in a metadacite, indicate presence of
hydrothermal fluids with high H+
/cations ratios, producing acid type alteration (Utada, 1980), at temperatures between
100 and ~300 °C. The hydrothermal alteration should have been subsequent to the maximum burial and the attainment
of epizonal metamorphism, and was very probably related with the posthumous activity of the Ordovician volcanic
arc. The widespread occurrence of retrograde diagenesis products (smectite, kaolinite, as well as interstratified Chl/Sm
and Chl/Vrm phases) in the anchizonal/epizonal metapelites of Sierra de Cochinoca-Escaya and Sierra de Rinconada
constitute, up to our knowledge, the first report of an extensive hydrothermal activity affecting not only the metavolcanic
rocks but also the Lower Ordovician sediments of northern Puna.