Paragénesis de zeolitas alcalinas en un afloramiento de basaltos olivínicos amigdaloides de Junín de Los Andes, Neuquén, Patagonia, Argentina

The optical characteristics, X-ray diffraction, unit cell, chemistry and crystallization conditions of an alkaline zeolite assemblage were studied. The zeolites have been found infi lling amygdales and in the matrix of Tertiary olivine basalts from Junin de los Andes, Patagonia. The secondary minerals (albite, pumpellyite, interestratifi ed chlorite/smectite (C/S), smectite, iddingsite, magnetite and hematite) are replacing phenocrysts of plagioclase, olivine and the matrix of the rocks. Zeolites are sodic, calco-sodic and potassic in composition. Na�chabazite, analcime, Ca-K phillipsite and paulingite are the fi rst zeolites that crystallized. They were followed by acicular-fi brous zeolites as natrolite/Na-gonnardite, thomsonite, mesolite and scolecite. The formation of the fi rst zeolites is consistent with low temperature (<100 ºC), low pressure (<0.1 GPa) and alkaline (pH 9�10) hydrothermal/geothermal meteoric fl uids, which were favored by tensional fractures. The activity of Si decreased during the alteration process and the temperature increased toward 200 ºC allowing the analcime formation followed by the equilibrium assemblage natrolite/Na-gonnardite-thomsonite-scolecite and mesoliteSe estudiaron las características ópticas, difracción de rayos X, celda unidad, características químicas y condiciones de cristalización de paragénesis zeolíticas alcalinas. Las zeolitas se encuentran rellenando amígdalas y en la matriz de basaltos olivínicos de edad terciaria de Junin de los Andes, Patagonia Argentina. Los minerales secundarios (albita, pumpellyita, interestratifi cados clorita esmectita (C/S), esmectita, iddingsita, magnetita y hematita) reemplazan a fenocristales de plagioclasa y olivino, y a los minerales de la matriz de las rocas. Las zeolitas son de composición sódica, calcosódica y potásica. Chabazita sódica, analcima, Ca-K phillipsita y paulingita son las primeras zeolitas en cristalizar y posteriormente lo hicieron las zeolitas aciculares-fi brosas natrolita/gonnardita sódica, thomsonita, mesolita y escolecita. Las zeolitas granulares se formaron a baja temperatura (<110 ºC) y baja presión (