Geochronology (40Ar/39Ar, K-Ar and He-exposure ages) of Cenozoic magmatic rocks from Northern Chile (18-22°S): implications for magmatism and tectonic evolutionof the central Andes

Abstract

K-Ar and Ar/Ar ages from magmatic rocks of northern Chile (18-22°S) describe duration and extent of the Tertiary and Quaternary magmatic evolution and date major tectonic events in northernmost Chile. This paper summarizes new K-Ar and Ar-Ar mineral and whole rock ages for intrusive rocks from the Precordillera, Tertiary ignimbrites and andesitic stratovolcanoes from the Western Andean Escarpment at 18°S (WARP) and the volcanic front. Intrusive rocks of the Precordillera (Quebrada Paguana, Quebrada Blanca, Quebrada Choja, Quebrada Guatacondo, Cerro Chandacolla) represent the Cretaceous to Eocene magmatic arc system and gave ages between 45 and 35 Ma. Younger ages on intrusive rocks are invariably caused by deuteric alteration. Ignimbrites of the Putani and Oxaya formations gave Ar-Ar sanidine ages around 24.2 to 24.8 Ma and 22.8 to 19.4 Ma, respectively. Andesitic stratovolcanoes, which directly overlie Oxaya ignimbrites east of the Western Cordillera gave ages of 20.3 Ma (Cordon Quevilque) to 9.0 Ma (Cerro Margarita). Samples from the Miocene to Pleistocene arc system on the Chilean Altiplano underlying the volcanoes of the active volcanic front have been dated between 10.5 to ~3 Ma. A widespread ignimbrite can be correlated from the Lauca basin to the Pacific coast and to the east to occurrences of near Pérez. Repeated Ar-Ar sanidine dating of the Lauca-Pérez-ignimbrite resulted in highly concordant ages of 2.71±0.25 Ma, 2.72 Ma±0.01 Ma, and 2.73±0.11 Ma. Rocks from the active chain (Volcanic Cordillera) gave ages younger than 0.9 Ma (Volcán Irruputuncu, Volcán Olca, Volcán Aucanquilcha, Volcán Ollagüe, Volcán Poruñita). These new data are used to constrain Miocene stratigraphy and tectonic movements as well as the timing of uplift and sedimentary response at the Western Andean Escarpment within the framework of the tectonic evolution of the Central Andes.