IGEBA   23946
INSTITUTO DE GEOCIENCIAS BASICAS, APLICADAS Y AMBIENTALES DE BUENOS AIRES
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
EDS COMPOSITIONAL CHARACTERIZATION OF PHENOCRYSTS IN BASALTS OF THE MIOCENE ALTA SIERRA VOLCANIC COMPLEX, SOMÚN CURÁ, NORTH PATAGONIA, ARGENTINA.
Autor/es:
CERREDO M.E.; M. B. REMESAL; CORDENONS P D.
Lugar:
Malargúe
Reunión:
Encuentro; 13° Encuentro del ?International Center For Earth Sciences; 2018
Institución organizadora:
CNEA
Resumen:
The Alta Sierra de Somún Curá Volcanic Complex (ASSVC) is located at central RioNegro Province, within the Oligo-Miocene Somún Curá Magmatic Province. It is made of several volcanic buildings encompassing 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 subordinated pyroclastic contributions. Semiquantitative mineral compositional data have been acquired by EDS measurements, both from the basic terms as well as for the trachyte facies. Semiquantitative mineral compositional data have been acquired by EDS measurements from the basic terms. This contribution is focused on the phenocryst/antecryst assemblages, from which some preliminary compositional trends are presented. The older subalkaline basalts to andesitic basalts bear mainly orthopyroxene phenocrysts (Wo 2-4; En 70-80; Fs 15-25; Ac 0-4) surrounded by thinrims of Mg-rich augite (Wo 30; En 50; Fs 20); occasional olivine microphenocrysts occur (Fo ∼ 50). Transitional subalkaline-alkaline basalts/trachybasalts to basaltic andesites/basaltic trachyandesites, show a richer phenocryst/antecryst association. The transitional subalkaline basic terms display 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). The alkaline transitional terms rounded clinopyroxene of similar composition is also found accompanied by clinopyroxene phenocrysts (Wo ∼ 34-43; En ∼ 40-53; Fs ∼ 12-15; Ac∼ 0-4). Orthopyroxene in the transitional facies occurs as relic crystals enveloped by clinopyroxene-olivine coronaes. Olivine microphenocrysts display a shift to more Mgrich compositions from the subalkaline terms to the alkaline series (i.e.: Fo from 55 to 75). The feldspar series, in turn, appear as phenocryst/antecrysts in the transitionalfacies. Whereas plagioclase phenocrysts in subalkaline transitional basic rocks range from calcic andesine to sodic labradorite, in the alkaline terms its composition is oligoclase.