Contact relationships of Tertiary volcanic units from the Andes of central Chile (33°S): a reinterpretation based on 40Ar/39Ar dating.

Abstract

Eight ⁴⁰Ar/³⁹Ar ages of rocks from two different areas of the Andes at ca. 33°S, where volcanic units separated by unconformities are exposed, are reported and discussed. These areas are Cuesta de Chacabuco at the northern prolongation of the Central Valley and Cerro Las Ollas at the western part of the Andean Range. In the Cuesta de Chacabuco area, the Chacabuco unit which consists of basalts, basaltic andesites, and continental sedimentary rocks is unconformably covered by pyroclastic flows of the Algarrobo unit, an eroded volcanic complex. A ⁴⁰Ar/³⁹Ar plateau age (whole rock, basalt) of 28.8±0.3 Ma was interpreted as the best estimate for the age of the volcanism of the Chacabuco unit whereas plateau ages obtained on amphibole crystals from basaltic andesite and andesitic dyke for the Algarrobo unit were 19.6±0.3 and 18.6±0.4 Ma, respectively. Thus, a hiatus on the order of 8.6 Ma exists between these two units. The Cerro Las Ollas area has been a disputed case in a long-standing discussion concerning the nature of the contact separating the Abanico and Farellones formations in the Andes of central Chile. The unconformable contact exposed there has been interpreted either as a folding and erosional unconformity or as a tectonic contact caused by 'décollement'. New ages obtained on biotite from tuffs fail to show a break in time between the rocks exposed below and above the disputed contact indicating that the tuffs belong to a same volcanic event. The best estimate for the age of this event is a ⁴⁰Ar/³⁹Ar mean weighted age of 20.1±0.1 Ma. Consequently, the unconformity present at Cerro Las Ollas may be considered to be of intraformational character.