Isotopic features (Sr-Nd) of the Pepe Núñez Alkaline Suite (Late Cretaceuos to Cenozoic) in the southern extreme of the Paraná Basin (NW Uruguay)

Abstract

The Pepe Núñez alkaline suite represents the first record of subvolcanic rocks of alkaline affinity in the Norte Basin (southern extreme of the Paraná Basin, Uruguay) [1]. These Late Cretaceous to Cenozoic occurrences comprise nine subvolcanic plugs (Ne-tephrites, alkali trachybasalts, and alkali basalts) that cross-cut the tholeiitic basaltic pile of the Paraná Magmatic Province [2]. This work presents Sr-Nd isotopic data from 30 representative samples to characterize their petrogenesis. Analyses were performed by TIMS (Sr) and ICP-MS (Nd). Initial 87Sr/86Sr (Sri) ratios range from 0.7038 to 0.7044, εNd(i) values range from -0.17 to +1.54. Depleted mantle model ages (TDM) vary between 663 Ma and 885 Ma. Geochemically, the samples form a tight cluster plotting within the Mantle Array, overlapping with the OIB (Ocean Island Basalts) field. Notably, the isotopic signature of these alkaline rocks is clearly distinct from the Low-Ti basalts of the Paraná Magmatic Province, showing no evidence of the significant crustal contamination typically observed in the tholeiitic sequences. The Ne-tephrites show slightly more depleted signatures (εNd(i)= +0.73 to +1.54) compared to the alkali basalts and trachybasalts (εNd(i)= -0.17 to +1.08). The data suggest that the Pepe Núñez Alkaline Suite originated from a mantle source with OIB-like affinity (resembling FOZO-PREMA, or metasomatized SCLM), likely related to partial melting of metasomatized lithospheric mantle or asthenospheric components that ascended rapidly, avoiding interaction with the continental crust.