New K-Ar ages on retroarc basalts of Mendoza-La Pampa

BERTOTTO G.W.; ORIHASHI, Y.; NAGAO, K.; MOTOKI, A.

Buenos Aires

Otro; Segundo encuentro científico del ICES; 2006

Comisión Nacional de Energía Atómica-Embajada de Italia en Argentina

Retroarc basaltic volcanism in the extra-Andean zones of Mendoza and La Pampa has been active since the Oligocene-Miocene up to prehistoric times. During previous studies it has been recognized five eruptive epochs of highest activity, calling them Palaocolitense (Oligocene-Miocene), Coyocholitense (Miocene), Chapualitense (upper Pliocene–lower Pleistocene), Puentelitense (Pleistocene) and Tromenlitense (Holocene). Here we present new ages determinations (K/Ar-whole rock) by “unspiked” technique, of rocks corresponding to localities Agua Poca (AP0) and Cerro Negro (NA2) in La Pampa, and Huanul (HU6) and De la Laguna (LA34) in Mendoza, all of them included at the easternmost border of the extra-Andean retroarc to this latitude. The ages obtained are 0.64 ± 0.04 Ma (AP0); 0.84 ± 0.05 Ma (HU6); 3.41 ± 0.2 Ma (NA2); 14.87 ± 0.87 Ma (LA34). Previous K-Ar analyses (whole rock) indicated ages of 0.6 ± 0.1 Ma for Agua Poca volcano, and 0.4 ± 0.1 Ma for a basaltic lava flow of Punta de la Barda (La Pampa). The rocks studied are alkaline basalts and basanites, that may have originated through low degrees of partial melting from enriched or primitive mantle sources. They present a trace element pattern with variable enrichment of incompatible elements (LaN/YbN=17.8−5.3). The oceanic plate that subducts beneath South America may be responsible for the behaviour of the incompatible elements, the content of which varies according to the geographic location of the studied rocks. Also, in the northern part of the study area, the presence of incompatible elements varies with time; the rocks of Pliocene-Pleistocene age have incompatible element ratios with arc signature, while the rocks of the Miocene De la Laguna volcano show cratonic affinity. These arc signature of the Pliocene-Pleistocene rocks can have been caused by slab fluids added to the mantle wedge at the time of subhorizontal subduction of the Nazca plate in the late Miocene.