Inicio  /  Andean Geology  /  Vol: 22 Núm: 2 Par: 0 (1995)  /  Artículo
ARTÍCULO
TITULO

Magma evolution within the accretionary mafic basement of Quaternary Chimborazo and associated volcanos (Western Ecuador)

Rolf Kilian    
Ernst Hegner    
Steven Fortier    
Muharrem Satir    

Resumen

ABSTRACT. Miocene to Holocene volcanic rocks of Chimborazo and associated volcanoes in the Western Cordillera of Ecuador range in composition from basaltic andesite to rhyolite. They were erupted through Cretaceous accretionary mafic basement. Three calk-alkaline magmatic trends can be distinguished in the Pleistocene and Holocene volcanic rocks. They are interpreted to reflect magma evolution within the immature continental crust. Early Pleistocene and Late Pleistocene volcanic rocks range in composition from basaltic andesite to dacite and are characterized by a moderate increase in alkalinity and little variation in the K/Rb, Ba/La, Th/Ta, and Sr/Y ratios. They show distinct geochemical differences that are attributed to variable proportions of lower crustal melts in the mafic end members. Volcanic rocks have lower 143Nd/144Nd ratios (0.5128 to 0.5129) than the basement, suggesting an origin of these isotopic characteristics from subduction modified mantle. Chemical modelling suggests that Late Pleistocene basalts may contain about 12% more of a lower crustal melt than Early Pleistocene parental melts. The development within both magmatic suites is explained by shallow level magma evolution. Other medium-K to high-K Pleistocene andesites may have been produced by mixing of a low-K basaltic melt generated from subduction-modified mantle and a high silica partial melt from the lower crust containing rutile and garnet. Partial melting of lower crust is supported by high Sr/Y (~200), Th/Ta (~42), and La/Yb ratios (~60) in the high-K andesites. 143Nd/144Nd ratios (0.51292) and delta 18 O values (-+9%0) in the high-K andesites are indistinguishable from those in Cretaceous accretionary basement and support a genetic relationship. RESUMEN. La evolucion magmatica dentro del basamento acrecionario mafico del volcan Chimborazo y volcanes asociados del Cuaternario (Ecuador Occidental). Las rocas volcanicas miocenas a holocenas del Chimborazo y volcanes asociados en la Cordillera Occidental del Ecuador varian en composicion de andesita basaltica a riolita. Fueron extruidas a traves de un basamento acrecionario mafico cretacico. Se distinguen tres tendencias de diferenciacion magmatica calco-alcalina en las rocas volcanicas pleistocenas y holocenas. Se las interpreta como indicativas de evolucion magmatica dentro de la corteza continental inmadura. Las rocas volcanicas del Pleistoceno temprano y tardio varian en composicion de andesita basaltica a dacita, y estan caracterizadas por un moderado incremento de alcalinidad y una pequena variacion de las razones K/Rb, Ba/La, Th/Ta y Sr/Y. Muestran diferencias geoquimicas distintivas que se atribuyen a proporciones variables de fundidos de la corteza inferior en los miembros extremos maficos. Las rocas volcanicas tienen razones de 143Nd/144Nd (0.5128 a 0.5129) menores que el basamento, lo que sugiere que el ongen de esta caracteristica isotopica reside en un manto modificado porsubduccion. El modelamiento quimico sugiere que los basaltos del Pleistoceno tardio pueden contener cerca de 12% mas de fundidos de la corteza inferior que aquellos del Pleistoceno temprano. El desarrollo de ambas series magmaticas se explica por evolucion magmatica a niveles poco profundos. Otras andesitas pleistocenas de mediano a alto K pueden haberse producido por mezcla de un fundido basaltico de bajo K generado en un manto modificado por subduccion, y un fundido parcial rico en silice de corteza inferior que contenga rutilo y granate. La fusion parcial de la corteza inferior esta apoyada por altas razones de Sr/Y (~200), de Th/Ta (~42), y de La/Yb (~60) en las andesitas de alto K. Las razones de 143Nd/144Nd (~0.51292) y los valores de delta 18 O (~+-9%0) en las andesitas de alto K, son indistinguibles de aquellas del basamento acrecionario cretacico, y sugieren una relacion genetica entre ellas.

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