The Miocene Magmatism in the Malargüe and Chos Malal Fold and Thrust Belts

Abstract

Late Oligocene to Pliocene magmatism at the latitudes of Malargue and Chos Malal fold and thrust belts, in the Neuquen Basin, is distributed from the main Andean axis to the retroarc zone. While arc magmatism maintained relatively similar compositional and geochemical features during late Oligocene to Pliocene times, major variations are seen in volcanic sequences developed in the retroarc zone, due to the development of a shallow subduction regime by mid-late Miocene times. Thus, late Oligocene-early Miocene period is characterized by an extensional regime that conditioned mainly tholeiitic magmas in the main Andean axis and alkaline intraplate magmas in the retroarc zone. Early-late Miocene marks a change to compressional tectonics in the Andean margin. Main arc magmatism showed a change to clearer arc, calc-alkaline signature, while retroarc magmatism showed the progressing input of slab-derived products as the shallow subduction regime triggered the eastward migration of the asthenospheric wedge. Thus, arc-derived lavas expanded into the mid to far retroarc zone (~69° 30″ to 68° 30″) with arc-like andesitic to dacitic compositions. With the progressive influence of the shallow subduction regime, arc-derived products reached almost 500 km away from the Chilean trench at ∽36° S by latest Miocene-Pliocene. By the middle Pliocene, re-steepening of the slabconditioned extensional tectonics that favored a widespread alkaline volcanism in the present-day Payenia retroarc and rhyolitic calderas in the main arc zone. After ∽3.5 Ma, retroarc magmatism at the latitudes of the Malargue and Chos Malal fold and thrust belts lack of clear arc-related geochemical features.