PHENOCRYSTS IN BASALTS OF THE MIOCENE ALTA SIERRA VOLCANIC COMPLEX: MINERAL COMPOSITION. SOMÚN CURÁ MAGMATIC PROVINCE, NORTH PATAGONIA, ARGENTINA

REMESAL, M B,; CERREDO M. E.; SALANI F.M.,

Mendoza, San Rafael

Encuentro; 14° Encuentro del "International Center For Earth Sciences"; 2019

The Alta Sierra de Somún Curá Volcanic Complex (ASSVC), in central Rio Negro Province, belongs to the Oligo-Miocene Somún Curá Magmatic Province. ASSVC is composed of several volcanic buildings spanning the basic to mesosiliceous realm and including both subalkaline to alkaline terms. The Middle to Late Miocene ASSVC is mostly a basalt to trachyte assemblage, dominated by basaltic lava flows with minor pyroclastic contributions.Semiquantitative EDS measurements were performed on phenocrysts of the basaltic rocks. Some preliminary characteristics of the ferromagnesian minerals are mentioned:-Orthopyroxene phenocrysts, found in the older subalkaline basalts to andesitic basalts, show an average composition of Wo 2-4; En 70-80; Fs 15-25; Ac 0-4, surrounded by thin rims of Mg-rich augite (Wo 30; En 50; Fs 20); occasional olivine microphenocrysts occur (Fo ~ 50).-A richer phenocryst/antecryst association is displayed by the transitional subalkaline-alkaline basalts/trachybasalts to basaltic andesites/basaltic trachyandesites. The phenocryst assemblage shows outstanding clinopyroxenes with clear disequilibrium features (rounded, corroded outlines, etc.,) and Mg-rich augitic compositions (Wo ~ 33-40; En ~ 36-50; Fs ~ 15-25; Ac~ 0-4). -Finally, in the alkaline transitional terms rounded clinopyroxene crystals occur along with clinopyroxene phenocrysts of average composition Wo ~ 34-43; En ~ 40-53; Fs ~ 12-15; Ac~ 0-4.