EOCENE-MIOCENE LONGITUDINAL DEPRESSION AND QUATERNARY VOLCANISM IN THE SOUTHERN ANDES, CHILE (33-42.5-DEGREES-S) - A GEOCHEMICAL COMPARISON

The Southern Andes Eocene-Miocene volcanism comprises a series of pale o-volcanic belts striking N-S to N10 degrees E. Along the longitudinal depression of central-south Chile, remnants of this volcanism are dis tributed between about 33 degrees and 42.5 degrees S. Basalts and basa ltic andesites predominate north of 37 degrees S, while andesites and dacites do it south. This general trend is opposite to that of the Sou thern Volcanic Zone (SVZ) Quaternary volcanism (33-46 degrees S), wher e andesites and dacites predominate north of 37 degrees S and basalts and basaltic andesites do it south of this latitude. Except for those Eocene-Miocene basalts and basaltic andesites found at 35.7 degrees S, basaltic rocks from the Longitudinal Depression Volcanic Belt are sim ilar in major and trace elements, and in Sr-, Nd-, and Pb-isotopes to the so-called type-1 (incompatible element depleted) Quaternary minor eruptive centers basaltic rocks from the central province (37-41.5 deg rees S) of the SVZ. The 35.7 degrees S basaltic rocks from the longitu dinal depression have REE patterns that mimic those of some ocean floo r basalts. The Sr-and Nd-isotope compositions of the type-1 Quaternary minor eruptive centers and basaltic rocks from the 33-33.5 degrees S area of the Longitudinal Depression Volcanic Belt are similar to those of OIB from the Juan Femandez archipelago and E-MORB rocks from the C hile Ridge. On the contrary, basaltic andesites and dacites from the 4 1.5-42.5 degrees S area of the belt are significantly enriched in radi ogenic Sr. Andesites and dacites from the 37.5-39 degrees S area are g eochemically and isotopically similar to Quaternary andesites and daci tes from the Nevados de Chillan Volcanic Group (36.8 degrees S), which are one of the isotopically most primitive andesites and dacites from the SVZ of the Andes.