Anders Scherstén

Back-arc volcanism in the western Argentinian provinces of Mendoza and Neuquen has been widespread from the Miocene to historic times. We present a detailed investigation of profiles through two of the major Miocene volcanic areas of the region, the neighboring Huantraico and La Matancilla plateaus, including new Ar-40/Ar-39 age results of major and trace elements as well as Nd, Sr and Pb isotopic data. Four million years of eruptions from 24.4 +/- 0.3 Ma (2 sigma) of alkali olivine basalts with OIB-type incompatible trace element enrichments at La Matancilla (similar to 36.50 degrees S) provide evidence for the presence of back-arc mantle devoid of subduction-related components. In contrast, the lower Huantraico lavas (similar to 37.30 degrees S) require an atypical back-arc mantle, almost devoid of arc-like components (e.g. low La/Ta = 15-18 and Ba/La = 12-18), but with a more depleted isotopic signature (e.g. Sr-87/Sr-86, 0.7033-0.7037) than observed elsewhere in the Andean back-arc. The Lower to Upper Series development in the Huantraico sequence represents a gradual change from basaltic to trachyandesitic back-arc lavas with a weak but temporally increasing arc geochemical signature (e.g. La/Ta = 15-21; Ba/La = 12-45), which is accompanied by Sr, Nd and Pb isotopic compositions approaching present day values of the Andes arc. The compositional change is accompanied by a gradually decreasing role for garnet in the mantle source, a decreasing degree of melting, but also simultaneously increasing influence from subducted fluids, probably as the slab geometry changes through time. The volcanism at Huantraico ceased when a flat slab was established around 15 Ma. (C) 2013 Elsevier B.V. All fights reserved.