PETROLOGY & GEOCHEMISTRY OF THE EL NEGRILLAR VOLCANIC FIELD, NORTHERN CHILE

The El Negrillar or Negros de Aras volcanic field comprises a group of Pleistocene cinder cones and lava flows in northern Chile that lie to the west of the main Central Volcanic Zone arc-front volcanoes (~30 km north of Socompa; ~25 km northwest of Cerro Pular). Samples were collected from at least9 different flow units and analyzed for major and trace element using ICP methods. Most of the lava flows are andesites, with one basaltic andesite and several dacites. Petrographically, the samples are sparsely phyric, with < 10% of microphenocrysts (< 1 mm) of olivine + plagioclase in the basaltic andesite, and plagioclase + clinopyroxene in the andesites and dacites. Major element compositions (e.g. 2-5 wt% MgO and 1.5-2.8 wt% K2O) are similar to the main arc-front volcanism. In contrast, incompatible trace element data show several features that mark the El Negrillarflows as compositionally distinct from the main arc-front volcanism. The El Negrillarflows have higher Sr (> 600 ppm) and higher Sr/Y (> 40), higher LREE/HFSE enrichment (e.g. La/Nb > 3), higher LREE-MREE (e.g. La/Sm > 5.2) and MREE-HREE (e.g. Dy/Yb > 2.4) enrichments and less-pronounced Eu-anomalies (Eu/Eu* > 0.85) compared to the main arc. About 15-30 km to the north & northeast, there are several isolated monogenetic centers (Cerro Tujle and Tilocalar Norte/Sul) that share similar petrographic and compositional characteristics with the El Negrillar volcanic field. These volcanic centers lie in an unusual tectonic position on the trench-side of the main Central Volcanic Zone arc-front volcanoes, and their locations are controlled by faulting associated with compressive deformation to the southeast of the Salar de Atacama.