MIDDLE JURASSIC VOLCANISM IN THE NORTHERN AND CENTRAL ANDES

Stratigraphical, petrographical, geochronological and geochemical data available suggest that the various segments of the western margin of South America experienced a different geodynamic evolution throughout the Jurassic. In the early Jurassic, the Northern Andes were character ized by an extension/tectonic regime whereas in the Central Andes subd uction led to the emplacement of a calc-alkaline magmatism. During the middle Jurassic (Bathonian to Oxfordian), an active subduction, perpe ndicular to the continental margin, generated a magmatic are (Misahual li/Colan are) along the Northern Andes. The volcanic products are medi um to high K calc-alkaline rocks composed of basaltic andesitic to rhy olitic lavas and acid pyroclastic rocks (ignimbrites, unwelded tuffs, volcanogenic sandstones and breccias). A new radiometric Ar-40-Ar-39 d ata gave an age of ca. 172 Ma for the Misahualli Formation of Ecuador, The Central Andes, in the southern coastal region of Peru, were chara cterized by an oblique convergence between the Phoenix oceanic plate a nd the continental margin of South America. This subduction produced a medium to high-K, calc-alkaline andesitic volcanic series (Rio Grande and Chala formations), comprising porphyritic basaltic andesites and emplaced as flows, intrusive rocks (dykes, 'sills' and 'stocks') and a cid pyroclastic rocks. New geochronological Ar-40-Ar-39 data confirm a n age of ca. 165 Ma for the andesitic basalt flows of the Chala Format ion.