Combined isotope study of the Ordovician Cerro de los Viejos Granite, La Pampa province, Argentina. Its derivation from the autochthonous, early Mesoproterozoic southwesternmost border of the Río de la Plata craton

Abstract

We have carried out the zircon U–Pb SHRIMP dating of the Cerro de los Viejos (CLV) Granite of southern La Pampa province, Argentina, which has yielded a concordia age of 468.0 ± 2.7 Ma, and determined the average Hf TDM model age of the dated zircon grains at 1491 Ma (Lower Mesoproterozoic; Calymmian), with an average epsilon Hf value of −3.13. Given the Ordovician (Darriwilian) crystallization age newly obtained for the CLV Granite, it becomes apparent that its previously known muscovite K–Ar Permian age of ca. 261 Ma would likely represent the age of the conspicuous shearing/mylonitization distinctive of this granitic body. Based on the new geochronologic and isotopic study of the CLV Granite, which also bears juvenile Rhyacian or “Transplatense” inheritance (aged at ca 2116 Ma, with average Hf TDM 2.4 Ga and average epsilon Hf value + 4.23) –an inheritance not known to occur in the neighbouring Patagonia or Pampia terranes–, as well as on comparisons with all the available geochronologic, isotopic, geophysical and geological data of the study region, in addition to broader tectonic considerations, we view the Ordovician CLV Granite as derived from the autochthonous, early Mesoproterozoic (Calymmian) southwesternmost portion of the Río de la Plata craton. The location of the CLV Granite would indicate that the peraluminous Ordovician magmatism of the southern La Pampa province, Argentina, straddles the Cambrian tectonic boundary between the Pampia terrane and the Río de la Plata craton. The CLV Granite would have been intruded in the immediate western continuation of the unexposed, E-W elongated upper crustal high/block recently identified by geophysical methods slightly to the east of the CLV Granite, the delineation of this block following the persistent sub-latitudinal structural fabric of the southern Río de la Plata craton, having also been reinforced by its probably late Middle Permian uplift coeval with the shearing of the CLV Granite.