New 40Ar/39Ar analyses constrain the formation of the volcanic succession of Sierra de Palaoco in the present back-arc of the Andean Southern Volcanic Zone (SVZ), near 36°S, to the Late Miocene and assigns them to the Huincán II Formation. The composition of major and trace elements, Sr, Nd and Pb isotopes of the Palaoco and nearby Río Grande rocks require a strong arc-like component in the mantle that is absent or weak in both Early Miocene (Fortunoso Group) and Pleistocene alkaline lavas (Llancanelo Group) erupted in the same area.
Weevaluate the relative roles of varyingmantle source compositions and crustal contamination in the generation of geochemically very different lavas from the Palaoco, Fortunoso and Río Grande volcanic fields, north of the Payún Matrú Volcano. The source for the Early Miocene Fortunoso(I) basalts was a OIB-type mantle devoid of subduction zone input. This type of OIB-like volcanic activity terminated due to a change from an extensional to a compressional tectonic regime. Towards the end of the Miocene renewed alkaline volcanism at Fortunoso
(II) display a transition to arc-type incompatible element enrichment. Shortly after the calc-alkaline Palaoco volcanism started with a very strong geochemical arc-signature including Ba/La ≈ 60 and La/Nb = 2–3. After a quiesence of 1 Ma the major part of the voluminous Late Palaoco basalts were erupted around 7.5 Ma over a few hundred ka. These are less enriched in Ba and Sr and have compositions like many Holocene rocks of the Southern Volcanic Zone. Isotopically the Fortunoso I and Palaoco rocks are distinct. Regional volcanism of the Charilehue, Huincán I and II mostly has a moderate arc-type enrichment indicating incipient arc developments.
However, Palaoco and La Brea at (c. 35°S) showfull geochemical arc-signature, andwe infer that a frontal arcwas established. The subsequent development in the Palaoco-Río Grande area encompasses renewed late Pliocene calc-alkaline low volume volcanic eruptions (Río Grande group) succeeded in the Late Pleistocene by alkaline OIB-type eruptions (Llancanelo group). In the light of the course of volcanism to the east, in the Nevado area, where late Miocene–Pliocene calc-alkaline volcanism was followed by Late Pliocene–Pleistocene alkaline volcanism.
We propose a scenario where the Nazca plate developed an eastwards widening flat slab from which the east dipping slab before the Late Pliocene translated from Palaoco to Nevado and subsequently retreated passing Río Grande in the Late Pliocene. Alkaline back-arc volcanism was active east of the arc-volcanism and expanded westwards during the Late Pliocene and Pleistocene.