Abstract

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 3.08 Ma 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 A 1-type granitoid affinities and are characterized by high silica and alkali contents (60-68 wt.% SiO 2; 8.7-10.8 wt.% Na 2O + K 2O), 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 them have the most radiogenic Sr values ever reported for a magmatic rock in the Meseta and even in the whole Neogene Patagonian Plateau Lavas province (( 87Sr/ 86Sr) o = 0.70556-0.70571; ( 143Nd/ 144Nd) o = 0.512603-0.512608). In addition, very high contents of strongly incompatible elements in the most evolved rocks, together with Sr isotopic ratios higher than those of coeval basalts, suggest the occurrence of open-system magmatic processes. Continuous fractional crystallization from a primitive basaltic source, similar to post-plateau coeval basalts, towards alkali granites combined with small rates of assimilation of host Jurassic tuffs (AFC) in a shallow magmatic reservoir, best explains the geochemical and petrographic features of the felsic rocks. Therefore, A 1-type magmatic rocks can be generated by open-system crystallization of deep asthenospheric melts in back-arc tectonic settings. In Central Patagonia, these ∼ 3-4 Ma old alkaline intrusions occur aligned along a ∼ N160-170 trending lineament, the Zeballos Fault Zone, stacking the morphotectonic front of one segment of the Patagonian Cordillera. Intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area, related to major regional tectonics occurring in possible relation with the collision of one segment of the Chile Spreading Ridge with the trench. © 2007 Elsevier B.V. All rights reserved.