Early Ordovician conodonts from the Ullum Volcanic Center, Argentine Precordillera: Biostratigraphy, geothermometry and preservation

Abstract

The present contribution deals with biostratigraphy, geothermometry and preservation of conodont elements recovered from blocks of the San Juan Formation at the core of the Ullum Volcanic Center, Argentine Precordillera. A collection of 1405 conodont elements corresponding to 22 genera and 34 species was recovered from five limestone samples. The biostratigraphic analysis allows us to document the Juanognathus variabilis Subzone of the Oepikodus evae Zone, updating previous interpretations. This is verified by the record of O. evae (Lindstr¨om), J. variabilis (Serpagli), O. communis (Ethington and Clark), and species that are typically abundant in the lower part of the identified zone. Thus, the oldest deposits that appear in the area correspond to the Floian Stage (Lower Ordovician). Moreover, regional and intercontinental correlations are carried out, showing strong links with the Oepikodus evae Zone from Baltoscandia, Newfoundland, and China, and the Reutterodus andinus Zone from the North America Midcontinent biostratigraphic schemes. Conodonts show a gradual CAI decrease away from the dacitic body contact, ranging from CAI 7 to CAI 6 across 40 m of stratigraphic thickness. The preservation of the conodont elements is directly related to the distance to the volcanic rocks, presenting abundant fractures and a stronger sugary bright with frosted surfaces in their adjacency. Mineral overgrowths are in general uniformly scarce in all studied samples, and siliceous and limonitic in composition. The degree of preservation of the conodont elements from Ullum is consistent with the hydrothermal-metasomatic processes that characterize the area. Moreover, when compared with paleotemperatures reached in the modelled thermal aureole, the predicted CAI temperatures, despite showing a gradual decrease, appear anomalously high. A feasible explanation to this fact would be high heat transfer by hydrothermalism; however, the conodonts from Ullum do not show most typical characteristics of this phenomenon.