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Inicio  /  Andean Geology  /  Vol: 15 Núm: 2 Par: 0 (1988)  /  Artículo
ARTÍCULO
TITULO

Tectonismo sinplutonico de dioritas jurasicas al sur de Valparaiso: datos U-Pb sobre la 'fase Quintay'

Estanislao Godoy    
Werner Loske    

Resumen

RESUMEN. Tres fracciones granulometricas de circones en una diorita de dos piroxenos (charnoquita 'precambrica' segun estudios anteriores), que aflora en Laguna Verde, y dos fracciones de su equivalente neisico de Punta Curaumilla, indicaron edades concordantes de 160 y 164 Ma. Una errorcrona Rb-Sr de cuatro puntos, obtenida en la diorita, tambien entrego un valor semejante (156,3+/-1,2 Ma). En una fraccion de circones de granito porfidico, recolectado en Caleta Barco, se obtuvo una edad U-Pb concordante (309 Ma). Estudios anteriores asignaron edades silurico-devonicas a rocas correspondientes a las dos ultimas localidades, basados en datos incompletos obtenidos por el mismo metodo y denominaron fase Quintay a una fase orogenica que habria ocurrido en dicho lapso. Los datos radio metricos aqui presentados y las caracteristicas texturales del neis dioritico son, en cambio, mas bien compatibles con un emplazamiento sincinematico de la diorita, a fines del Jurasico medio. Apoyan, por otra parte, la conclusion alcanzada por otros autores mediante dataciones Rb-Sr, en cuanto a que las edades U-Pb pre-carboniferas, antes mencionadas, pueden corresponder a edades discordantes, ligadas a circones heredados. El magmatismo granitico en este sector costero de Chile central, en el cual se ha buscado, insistentemente, evidencia de corteza continental antigua, habria comenzado recien a partir de la Carbonifera, reactivandose en el Jurasico. ABSTRACT. U-Pb dating of three zircon fractions from a two-pyroxene diorite from Laguna Verde, previously considered a Precambrian charnockite and of two fractions from its gneissic equivalent at Punta Curaumilla systematically yield concordant ages (160-164 Ma). A mineral-whole rock Rb-Sr errorchron from the diorite is also compatible with this age (156.3+/-1.2 Ma). On the other hand, a concordant U-Pb 309 Ma age was obtained in zircons from porphyric granite at Caleta Barco. Previous incomplete U-Pb determinations on the two last localities have been used to infer a Devonian orogenic phase (Ouintay phase). Both the new radiometric data and textural evidence, however, favor a Late Jurassic synkinematic emplacement of the diorite. Present U-Pb data, on the other hand, support the conclusion reached through Rb-Sr dating by other authors, regarding the interpretation of pre-Carbonilerous ages. They most likely represent discordant ages linked to inherited zircons. The granitic magma contribution to the building of continental crust in this area started, therefore, in the Carbonilerous and was reassumed during the Jurassic.

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