Geochemical variations in Cenozoic back-arc basalts at the border of La Pampa and Mendoza provinces, Argentina

BERTOTTO, G. W.; CINGOLANI, C. A.; BJERG, E. A.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elsevier

Año: 2009 vol. 28 p. 360 - 373

0895-9811

Retroarc volcanism was active in central-western Argentina (provinces of Mendoza and La Pampa) from the Miocene through historic times. The rocks of 39 monogenetic volcanoes located in this area were studied in order to define their geochemical characteristics. The dominant rock texture is porphyritic, with intergranular, pilotaxitic and hyalophitic groundmasses. The most frequent phenocrysts are olivine followed by olivine-plagioclase-clinopyroxene. The most abundant rocks are trachybasalts, followed by basalts and basanites, all of them alkaline. Most of the studied rocks classify as basic, their SiO2 content varies between 42.3 and 51.2 wt.% and they display variable enrichment of incompatible elements (La/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 and age 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 variations probably reflect the eastward migration of the slab fluid metasomatism in response to a reduction of the angle of subduction of the slab in the late Miocene.