THE AGUA DE LA PIEDRA VOLCANIC COMPLEX: LITHOLOGIC ASSEMBLAGE, WHOLE ROCK AND MINERAL CHEMISTRY

SALANI, F.M; CERREDO, M. E.; CORDENONS, P. D.; REMESAL, M. B.

Buenos Aires

Encuentro; 10 encuentro internacional E-ICES-10; 2014

CNEA

The Agua de la Piedra Volcanic Complex (APVC) is an alkaline bimodal assemblage of pyroclastic and lava flows located at the SW area of the Oligo-Miocene Somún Curá Magmatic Province, northern extrandean Patagonia. This contribution deals with APVC description, its main facies, petrographic and chemical characterization, and its identity within the framework of the Cenozoic volcanism of North Patagonia. The APVC is made of three main facies: 1) a pyroclastic association of trachytic pumiceous flows, tuffs and interbedded tuffaceous sandstones; 2) trachyte and rhyolite lavas and 3) basaltic lavic facies. The APVC volcanic series defines a bimodal alkaline trend with a gap between 52 and 66 SiO2 wt %, as commonly found within the Somún Curá province. The trachyte lavas constitute the core of the Complex; porphyric trachytes bear phenocrysts of alkaline feldspars (anortoclase and sanidine), alkaline clinoamphibole (riebeckite-arfverdsonite) and biotite, set in a groundmass of alkaline feldspar microliths, accompanied by accessory amounts of zircon, apatite and magnetite. Rhyolite lavas are subordinated in volume; rocks are porphyric with quartz, sanidine and minor biotite phenocrysts immersed in a micrograined to granophyric groundmass. Na-rich alkaline microliths, tiny Fe-Na clinopiroxenes and clinoamphibole (riebeckite-arfverdsonite) make the bulk of the mesostasis along with minor magnetite, apatite, zircon and Nb-bearing phases. The basaltic facies partially envelopes the silicic association, mainly towards the W and N of the Complex; basaltic and trachybasaltic terms are distinguished within this facies. The former are typically aphyric composed of intermediate Fe-Mg olivine, diopsidic clinopyroxene, andesine microliths and minor ilmenite. Trachybasalts are porphyric with iddingsitised olivine, Ti-augite and labradorite phenocrysts, accompanied by plagioclase crystals with spongy cores and fresh rims. The groundmass is made of ternary Na-rich feldspar microliths, augite and minor ilmenite. The new semiquantitative mineral compositions along with bulk chemical information are powerful tools to characterize the APVC evolutionary trend.