The Miocena volcánica belt of the northern Precordillera, San Juan and La Rioja provinces, Argentina

Abstract

The existence of a thick sequence of volcanites and volcaniclastic rocks deposited during the middle Miocene in the north of Precordillera is considered in this paper. These rocks have been assigned to the Triassic in early papers, nevertheless new radiometric data here presented point out a Miocene age by the volcanites (17,6 ± 0,5 Ma and 18,3 ± 0,7 Ma) and suggests their inclussions in the Cerro Morado Formation. In the analized area the mentioned unit lays unconformably on Tertiary eolian sandstones of the Vallecito Formation (Miocene) and shows at the top a tectonic relations with shales belonging to the Yerba Loca Formation (Ordovician). Nevertheless to the south, in the Huaco anticline, Cerro Morado Formation is cover by miocene conglomerates and sandstones of the Cauquenes Formation (Upper Miocene ?). On the basis of its lithological features, the Cerro Morado Formaton is divided in four facies associations. The association I is composed of volcanics flows and breccias that mark the phase of maxima volcanic activity in the region. The association II, made up of sedimentary breccias and scarce volcanic flows, points out an ameloration of the volcanism, the developed of pyroclastic flows and the existence of talus and alluvial fan deposits. The association III is dominated by conglomerates and breccias formed by high concentration fluvial flows and debris flows deposited in alluvial fans and proximal braided rivers. Finally association IV comprises conglomerates and sandstones mainly formed in braided rivers by diluite flows The existence of Neogene volcanites in north of Precordillera reinforces the hypothesis of arc volcanic migration as a consequence of a progressive decrease in angle of subduction of the Nazca plate during the Miocene.