MIO-PLIOCENE MAGMATIC VARIABILITY IN THE CENTRAL PATAGONIA BACK-ARC REGION (47º5'S)

The thick alkali basaltic piles forming the Meseta del Lago Buenos Aires (MLBA, 47.5ºS) and Meseta Chile Chico (MCC, 46.5ºS), which belong to the Neogene Plateau Lavas of southern Patagonia, are thought to result from the ascent of subslab asthenospheric magmas through slab windows opened due to the breakoff of the Nazca Plate. Calc-alkaline lavas (Hb-bearing andesites; 53-58 wt% SiO2; 4.7-6.6 wt% Na2O+K2O; La/Nb: 2.6-3.0) displaying whole rock K-Ar ages of ca. 16-14 Ma were recognized at the top of the Miocene Zeballos Group continental molasse, over which are spilled elsewhere the basal flows of MLBA (ca. 12 Ma). The occurrence of this Middle Miocene volcanism indicates that subduction-related magmatism in the region started at the time when the first segments of the Chile Spreading Ridge collided with the tip of South America (55ºS). Furthermore, these lavas could represent the contaminant accounting for the transitional signature (La/Nb>1; TiO2<2 wt%) observed in several younger basalts of MLBA and MCC. The occurrence of these lavas put temporal constrains on the evolution of Patagonian Cordillera (Lagabrielle et al., this issue), and documents the continuity of subduction-related volcanism since the Jurassic in the Patagonian continental border. In addition, the occurrence of Pliocene alkali syenites (3.28±0.10 Ma; 4.9 wt% K2O; 5.2 wt% Na2O), intruding the youngest MLBA basalts (3- >1 Ma), could indicate that the Pliocene magmatism in the Meseta is more complex than a typical slab window magmatism, as previously accepted.

The magmatic variability found in these back-arc rocks demonstrates the complexity and heterogeneity of magma sources beneath Patagonia. Indeed, these lavas could have been generated from astenospheric enriched- and from hydrated mantle wedge- sources, and later fractionated and contaminated by crustal melts.