Bimodal back-arc alkaline magmatism after ridge subduction: Pliocene felsic rocks from Central Patagonia (47°S)

© 2007 Elsevier B.V. Volumetrically minor microsyenites, alkali microgranite and related trachytic dykes intrude early Pliocene OIB-like alkali basaltic and basanitic flows of the Meseta del Lago Buenos Aires in Central Patagonia (47°S-71°30'W), and occur together with scarce trachytic lava flows. Whole-rock K-Ar ages between 3.98 and 3.08Ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post-plateau basaltic sequence that they intrude, during a bimodal mafic-felsic magmatic episode devoid of intermediate compositions. Chemically, these rocks have A1-type granitoid affinities and are characterized by high silica and alkali contents (60-68wt.% SiO2; 8.7-10.8wt.% Na2O+K2O), major and trace elements patterns evidencing evolution by low-pressure fractional crystallization, and Sr and Nd isotopic signatures similar to those of coeval basalts ((87Sr/86Sr)o=0.70488-0.70571; (143Nd/144Nd)o=0.512603-0.512645). Nevertheless, some of the