POST-DEPOSITIONAL EVOLUTION OF THE ORDOVICIAN SUCCESSIONS OF NORTHWESTERN ARGENTINA

NIETO, F.; DO CAMPO, M.

Granada

Congreso; XVI International Clay Conference; 2017

Sociedad Esapañola de Arcillas y AIPEA

The thermal post-depositional evolution of metapelitic and metavolcanic rocks of the upper Cambrian-Ordovician succession of the Central Andes of northwestern Argentina was estimated through clay mineral analysis, Kübler index, SEM-EDS study of selected samples and chlorite geothermometry [1]. The study area comprise five representative regions in Cordillera Oriental and Puna, that are from east to west: Sierra de Santa Victoria (Nazareno and Santa Victoria river areas), Cordón de los Siete Hermanos (Yavi), Sierra de Cochinoca-Escaya and Sierra de Rinconada. Based on KI values and occurrence (or absence) of slaty cleavage the studied rocks can be divided in three groups:1) Almost all the mudstones from Sierra de Santa Victoria show KI values in the range of deep diagenesis-low anchizone and a rough bedding-parallel microfabric (S0). The clay mineral assemblages identified in these metapelites illite?muscovite + chlorite  kaolinite, or illite-muscovite  Chl/Sm  I/Sm are in agreement with the diagenetic-anchizonal grade indicated by KI values. 2) To the west, slates from Yavi depict KI values in the range of high anchizone-epizone (0.30-0.362) and rough anastomosing spaced cleavage (S1) parallel to the sedimentary layering (S0), or a crenulation cleavage (S1) oblique to S0. The clay mineral associations identified in these slates (illite-muscovite  kaolinite  chlorite  smectite) do not represent equilibrium assemblages, neither are they consistent with the KI values indicating high anchizone to epizone. Consequently smectite and kaolinite probably represent retrograde-diagenetic products Nieto et al., 2005]. 3) Finally, slates and metavolcanic rocks from Puna show KI values from high anchizone to epizone (0.23-0.352). In these slates a discontinuous crenulation cleavage (S1) oblique to S0 or an S1 cleavage, mainly sub parallel to the sedimentary layering (S0) is observed. Few samples of this group show clay-mineral assemblages comprising illite-muscovite and chlorite, in coherence with the anchizonal/epizonal grade indicated by KI values. On the contrary, most of the samples consist of complex clay-minerals assemblages comprising, in addition to illite-muscovite and rarely chlorite, diverse phyllosilicates like kaolinite, smectite, and various mixed-layers (Chl/Sm, Chl/Vrm and Chl/Mi) that are not stable in the anchizone and neither in the epizone. Consequently, these phases could not represent prograde metamorphic phases in these rocks. Furthermore, metaandesites from Sierra de Cochinoca-Escaya present strong evidences of hydrothermal alteration at optical microscope and SEM, such as phenocrysts of albite depicting dissolution voids, embayments filled with mixed-layered phyllosilicates, replacement by fine-grained white mica and sulfates of the alunite-jarosite family occurring in close association with goethite. Moreover, mixed-layered phyllosilicates form veinlets, sometimes associated with scarce anhedral quartz. Geothermometric calculations performed for the three chlorite-bearing samples yield temperatures from 124 to 199C for a mudstone from Nazareno (n= 6, average 138C) and values in the range from 179 to 339C (n= 18, average 273C) for Cordón de los Siete Hermanos . On the other hand, a wide range of temperatures from 151 to 322C (n= 28, average 231C) were obtained for chlorites from a slate of northern Puna. However, geothermometric calculations could not be accomplished for a significant number of chlorite analyses enclosed in the  < 0.1 p.f.u./Si < 2.5 p.f.u. window, that are outside the T range of applicability of Bourdelle?s thermometer, and thus probably formed at T >350C. The paleotemperatures obtained with the chlorite geothermometer, in coincidence with KI values, showed an E-W trend from diagenesis/low anchizone in the eastern localities, to high anchizone/epizone in the Puna, with intermediate values in locality of Yavi. 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 (smectite, kaolinite, as well as interstratified Chl/Sm and Chl/Vrm) in most of these rocks and the occurrence of jarosite in a metadacite, indicate hydrothermal fluids with high H+/cations ratios, producing acid type alteration, 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. This is, up to our knowledge, the first report of an extensive hydrothermal activity affecting not only the volcanic rocks but also the Lower Ordovician sediments of northern Puna.