Abstract

The Quebrada Blanca igneous unit (Upper Permian-Lower Triassic) intruding the crystalline basement of the Sierra de La Huerta, Western Sierras Pampeanas, is the most differentiated member of a group of similar age and magmatic features distributed in the cited range. it comprises a hypabyssal rhyolite porphyry, a hydrothermal-intrusive breccia wherein two facies with contrasting texture and composition were distinguished, and felsite/rhyolite dikes. This association defines the root zone of a diatreme-type breccia which has been exposed by erosion. Breccia formation was associated with crystallization of the porphyry rhyolite, explosive brecciation, and exsolution of magmatic hydrothermal fluids which broke off and carried upward fragments of the solidified porphyry carapace and metamorphic wall rocks. The rhyolite porphyry was emplaced afterwards probably occupying the feeder conduit. The geochemistry data indicate a high-silica, subalkaline, high-K calk-alkaline and metaluminous to slightly peraluminous affinity for the Quebrada Blanca igneous Unit. Large-ion lithophile elements contents are high whilst Sr, P and Ti contents are low. They show enrichment of LREE and HREE depletion (La/Yb N = 3.37 to 13.67). Geochemical characteristics are compatible with igneous rocks related to volcanic arcs. The studied rocks are tentatively correlated to the dacitic-rhyolitic upper section of the Choiyoi Group and could be assigned to the final stages of the continental magmatic arc developed along the western margin of Gondwana since the Late Carboniferous.