Se han obtenido edades 40Ar/39Ar (calentamiento por pasos en horno y mediante laser) en plagioclasas de secuencias volcanicas de dos secciones del Cretacico Inferior de la Cordillera de la Costa (FormacionArqueros) a la latitud de La Serena (=29°S). Debido a la alteracion parcial de los cristales de plagioclasa, se han observado espectros de edades perturbados en las medidas realizadas en horno. Sin embargo, en las determinaciones realizadas mediante laser en unos pocos granos de plagioclasa meticulosamente seleccionados se han obtenido edades plateau correspondientes a plagioclasa primaria, como lo evidencian los valores constantes de la relacion 37Arc¡/39ArK. Se han obtenido edades plateau de 114,1±0,5 Ma (muestra ARQ99-4), 111,3±0,9 Ma (muestra TC99-5a) y 91,0±0,6 Ma (muestra TC99-2) en diferentes coladas de lava, y de 84,3±1,3 Ma en un dique (muestra ARQ99-7). Estas nuevas edades 40Ar/39Ar, junto a las ya publicadas en Chile central, permiten constreñir mejor la duracion del evento magmatico extensional durante el Cretacico Inferior en la Cordillera de la Costa del centro y centro-norte de Chile. Estos datos estan de acuerdo con la existencia de una Provincia Magmatica del Cretacico Inferior (119-84 Ma), que habria comenzado con un breve e intenso evento magmatico en la zona central de la Cordillera seguido de pulsos magmaticos discretos hacia latitudes mas septentrionales.ABSTRACT:40Ar/39Ar age data (laser and furnace step heating) on plagioclase from Lower Cretaceous volcanic sequences from the Arqueros Formation in two sections of the Coastal Range at the latitude of La Serena (=29°S) have been obtained. Due to the partial alteration of plagioclase crystals, disturbed age spectra in the furnace experiments have been observed, whereas laser heating determinations involving a much smaller quantity of grains carefully selected, could display plateau ages corresponding to pure plagioclase, as demonstrated by a constant 37Arc¡/39ArK ratio. Plateau ages of 114.1±0.5 Ma (sample ARQ99-4), 111.3±0.9 Ma (sample TC99-5a), and 91.0±0.6 Ma (sample TC99-2) were found in lava flows, and 84.3±1.3 Ma on a dyke (sample ARQ99-7). These new 40Ar/39Ar ages, together with those previously published in central Chile, allow a constriction of the extensional magmatism during the Early Cretaceous in the Coastal Range of central and north-central Chile. All these data are in accordance with a long lived Early Cretaceous Magmatic Province (119-84 Ma), that could have started with a brief and huge magmatic event, mostly developed in the central part of the Coastal Range, followed by discrete magmatic pulses at further northern.40Ar/39Ar age data (laser and furnace step heating) on plagioclase from Lower Cretaceous volcanic sequences from the Arqueros Formation in two sections of the Coastal Range at the latitude of La Serena (=29°S) have been obtained. Due to the partial alteration of plagioclase crystals, disturbed age spectra in the furnace experiments have been observed, whereas laser heating determinations involving a much smaller quantity of grains carefully selected, could display plateau ages corresponding to pure plagioclase, as demonstrated by a constant 37Arc¡/39ArK ratio. Plateau ages of 114.1±0.5 Ma (sample ARQ99-4), 111.3±0.9 Ma (sample TC99-5a), and 91.0±0.6 Ma (sample TC99-2) were found in lava flows, and 84.3±1.3 Ma on a dyke (sample ARQ99-7). These new 40Ar/39Ar ages, together with those previously published in central Chile, allow a constriction of the extensional magmatism during the Early Cretaceous in the Coastal Range of central and north-central Chile. All these data are in accordance with a long lived Early Cretaceous Magmatic Province (119-84 Ma), that could have started with a brief and huge magmatic event, mostly developed in the central part of the Coastal Range, followed by discrete magmatic pulses at further northern latitudes.