Abstract

VATTUONE, María E. et al. Paragenesis of alkaline zeolite in an outcrop of amigdaloidal olivine basalt in Junín de los Andes, Nequén, Patagonia, Argentina. Rev. mex. cienc. geol [online]. 2008, vol.25, n.3, pp.483-493. ISSN 2007-2902.

The optical characteristics, X-ray diffraction, unit cell, chemistry and crystallization conditions of an alkaline zeolite assemblage -were studied. The zeolites have been found infilling amygdales and in the matrix of Tertiary olivine basalts from Junin de los Andes, Patagonia. The secondary minerals (albite, pumpellyite, interestratified chlorite/smectite (C/S), smectite, iddingsite, magnetite and hematite) are replacing phenocrysts ofplagioclase, olivine and the matrix of the rocks. Zeolites are sodic, calcosodic and potassic in composition. Na-chabazite, analcime, Ca-K phillipsite and paulingite are the first zeolites that crystallized. They were followed by acicular-fibrous zeolites as natrolite/Nagonnardite, thomsonite, mesolite and scolecite. The formation of the first zeolites is consistent with low temperature (<100 °C), low pressure (<0.1 GPa) and alkaline (pH 9-10) hydrothermal/geothermal meteoric fluids, which were favored by tensional fractures. The activity of Si decreased during the alteration process and the temperature increased toward 200 °C allowing the analcime formation followed by the equilibrium assemblage natrolite/Nagonnardite-thomsonite-scolecite and mesolite.

Keywords : alkaline zeolites; basalts; Junín de Los Andes; Patagonia.