BURIAL HISTORY AND TECTONOTHERMAL EVOLUTION OF THE CUYANIA TERRANE (ARGENTINE PRECORDILLERA) AS REVEALED BY CONODONT COLOR ALTERATION DATA

Thermal maturation of the lower Paleozoic carbonate platform from the Argentina Precordillera is a critical parameter to evaluate the geotectonic evolution of the Cuyania composite terrane. The conodont Color Alteration Index (CAI) of >7800 Cambrian to Silurian specimens allows the thermal analysis for 235 samples (~360 kg rock weight), representing 50 localities of the Precordillera. The regional pattern of CAI values shows an increase of paleotemperatures to the west and, less defined, to the south of the basin. This configuration records a gradual and continuous transition from diagenesis to low-grade metamorphism, which is apparently not controlled by any of the morpho-structural subdivisions of the Precordillera fold and thrust belt (i.e., Western, Central, or Eastern). In particular, the predominant component for the heating of the Western Precordillera was overloading by thrust sheets, resulting from the Devonian collision of Chilenia. Alternatively, the mafic-ultramafic bodies intruding the Western Precordillera produced narrow thermal aureoles, of about 2.5 times intrusion thickness, that prevailed over the later very low-grade metamorphism. Additionally, an Ordovician metamorphic event is recorded by means of conodont paleothermometry in the slope sedimentary succession of the Western Precordillera. Allochthonous conodonts from reworked deposits of the eastern carbonate platform with CAI 4.5–5 and autochthonous conodonts from the slope facies with CAI 3 allow for constraining the metamorphic age within the Paltodus deltifer and Lenodus variabilis zones (i.e., ca. 480–465 Ma). The allochthonous conodonts provenance, as demonstrated by sedimentary paleocurrents in the slope sequences, verifies this metamorphic event occurred in the eastern border of the Precordillera. Accordingly, the conodont data support an Ordovician instead of an argued Devonian collision for the Cuyania terrane with the proto-Andean margin of Gondwana, which is consistent with the earlier proposed, but still contentious, microcontinent model. The occurrence of allochthonous and autochthonous conodonts in the slope facies of the Western Precordillera provides a time constraint for the early accretion of the Cuyania terrane and its transition to an evolving foreland mountain system in the Ordovician Period.