K-Bentonite deposits in the argentine Precordillera: Characterization and new findings from the eastern Precordillera of San Juan

The rifting, drifting and eventual accretion of the Cuyania Terrane to the southwestern margin of Gondwana represents one of the most well-documented cases of the accretion of an allochthonous lithospheric fragment. Among the key lines of evidence supporting the proximity of Cuyania Terrane to the paleomargin of Gondwana is the occurrence of numerous K-bentonite levels interbedded with Ordovician sedimentary successions in both the Precordillera and the Famatina System. These volcanic ash fall deposits are interpreted as the result of Famatinian arc volcanism during the pre-collisional stage of Cuyania, when the remnant ocean separating the two lithospheric fragments was gradually closed. This contribution presents the first zircon U-Pb geochronological results from a K-bentonite level in the Eastern Precordillera of San Juan, specifically within an olistolith of limestone that was resedimented into the Rinconada Formation (Silurian-Early Devonian?). This discovery documents the first known occurrence of K-bentonite in the Sierra de La Rinconada, and the second within the Eastern Precordillera. The results yielded an age of 475 ± 8 Ma (Floian). The new information obtained is compared with the current knowledge of the spatial and temporal distribution of the Famatinian volcanism as well as with the existing biostratigraphic framework. Thus, the new age recorded for this K-bentonite level is broadly congruent with the temporal extent of Famatinian volcanism (ca. 479-463 Ma), and concordant with existing biostratigraphic information.