VOLCANIC RECORD OF THE SUBDUCTION OF AN ACTIVE SPREADING RIDGE, TAITAO PENINSULA (SOUTHERN CHILE)

During the Pliocene, the Taitao Peninsula was the locus of the subduct ion of the Chile active spreading ridge beneath the South American mar gin. New field data allow us to distinguish within the Taitao ophiolit e a genuine ophiolitic body of undetermined age and two volcanic and v olcaniclastic units. One unit shows evidence for deposition in a shall ow-water environment, and it unconformably overlies the Chilean contin ental basement. Its established Pliocene age demonstrates that it was deposited during the subduction of the Chile ridge. Lavas from both un its include enriched mid-oceanic ridge basalts (MORBs) together with N b-depleted basalts, andesites, and dacites. La/Nb ratios vary randomly from 1 to 4 in both units and are positively correlated with SiO2 con tent. These chemical characteristics are consistent with upper-crustal contamination of MORB-type basalts by the Chilean crust as already su ggested from isotopic data. We consider these volcanic suites to be th e result of eruptions of MORB magmas that originated from the downgoin g, shallowly buried, active spreading center and were contaminated dur ing their rise through the Chilean continental basement.