Revisión estratigráfica y consideraciones paleogeográficas de la tectofacies occidental de la Precordillera argentina

A synthesis of current knowledge on the western tectonofacies of the Argentine Precordillera points to the potential correlation of different lithostratigraphical units on the basis of lithology, facies analysis, petrographical characteristics and faunal content. Graptolites are shown to be particularly useful, not only in constraining ages, but also in identifying specific water masses more precisely and in determining the palaeogeography. Particularly important, is the recent finding of Ashgill, most probably Hirnantian graptolites, intimately associated with pillow lavas of the Alcaparrosa Formation. This new age rules out the hypothesis that these mafic volcanic rocks in western Precordillera were related to the rifting that separated the Precordillera from Laurentia. Moreover, contrasting lithofacies and graptolite biogeography indicate that strong differences existed between the Precordillera and Laurentia by the Late Ordovician, and suggest that the Precordillera was located in cool-water western Gondwana latitudes by then. Instead, the mafic volcanic rocks in the western Precordillera were related to Late Ordovician extension (post-orogenic relaxation), and the eventual separation of a para-autocthonous fragment of Grenvillian basement from the Precordillera terrane, known as Chilenia. Moreover, relatively new findings of Early Cambrian oceanic crust, tectonically included in the southern extent of the western tectofacies in Mendoza province, reinforce the postulated Cambrian separation between the Precordillera and Laurentia.