Subduction controls on Miocene back-arc lavas fromSierra de Huantraico and La Matancilla and new40Ar/39Ar dating from the Mendoza Region, Argentina

CHARLOTTE T. DYHR; PAUL H. HOLM; EDUARDO J. LLAMBÍAS; ANDERS SCHERSTÉN

LITHOS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2013 vol. 169 p. 67 - 83

0024-4937

Back-arc volcanism in the western Argentinian provinces of Mendoza and Neuquén 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 LaMatancilla plateaus, including new 40Ar/39Ar 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 (2σ) of alkali olivine basalts with OIB-type incompatible trace element enrichments at La Matancilla (~36.50°S) provide evidence for the presence of back-arc mantle devoid of subduction-related components. In contrast, the lower Huantraico lavas (~37.30°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. 87Sr/86Sr, 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.