Correlations between metapelitic zones and graptolite reflectance in Ordovician successions from northwestern Argentina depicting a complex postdepositional history

Abstract

Correlations between metapelitic zones and organic proxies of thermal maturity, like coal rank, vitrinite reflectance and color alteration index in conodonts, were developed several decades ago and have been widely applied with satisfactory results. In contrast, comparisons between graptolite reflectance and metapelitic zones defined by Kübler Index values, clay mineral assemblages and chlorite geothermometry are scarce. In this study, samples from Ordovician successions in northwestern Argentina, where graptolite reflectance had been previously measured, were analyzed for clay mineral assemblages using X-ray diffraction, Scanning Electron Microscopy with Backscattered Electrons, and Electron Probe. Additionally, the Kübler Index was determined, and a chlorite geothermometer was applied to compare the response of these proxies for diagenetic-metamorphic temperatures and thermal maturity. The studied material was selected from two sections: Los Colorados, in the western part of the Cordillera Oriental, which mainly comprises the Acoite Formation (Early-Middle Ordovician), and Muñayoc, in the eastern part of the Puna highland, corresponding to the Cochinoca-Escaya Magmatic-Sedimentary Complex (Early-Middle Ordovician). The samples coming from the Los Colorados section reveal a clay mineral assemblage consisting of dioctahedral mica, kaolinite, along with scarce chlorite-bearing mixed-layers and chlorite in few cases, exhibiting Kübler Index values indicative of diagenesis (≥ 0.52 Δ°2θ), and corresponding temperatures between 109 and 282 °C. In contrast, samples from the Muñayoc section present a clay mineral assemblage, comprising mainly dioctahedral mica illite, subordinate kaolinite and smectite, associated with chlorite in few samples; Kübler Index values agree with the epizonal zone (≤ 0.32 Δ°2θ), and temperatures ranging from 177 to 321 °C. Notably, these findings show improved correlation with the temperatures derived from graptolite reflectance when considering updated equations between graptolite and vitrinite reflectance. On the other hand, most of the samples from Muñayoc show non equilibrium clay assemblages, as smectite and kaolinite are not stable in the epizone. This fact, along with the absence of chlorite in several samples, is probably a consequence of the hydrothermal alteration that affected the Ordovician rocks in several localities of the Puna region. As any of these approaches has its own limitation factors and sources of uncertainty, the combination of inorganic and organic proxies provide a more robust methodology to outline low-temperature burial pathways. Moreover, the correlation between Kübler Index and equivalent vitrinite reflectance revealed higher geothermal gradients in Puna than in Cordillera Oriental.