IMPLICATIONS OF LATE MIOCENE VOLCANISM FOR CLOSING THE PANAMANIAN SEAWAY

El Valle volcanism represents the youngest volcanism of the Isthmus of Panama. We present sedimentological, stratigraphic, geochronological, and geochemical data that suggest a relationship between the onset of El Valle volcanism, the volcaniclastic deposits of the Gatun Formation and the shallow marine deposits of the Chagres Formation. The significance of this relationship is that this young volcanism may have blocked the final —and shallow— Central American seaway in the late Pliocene. Composed by a thick sequence of ~ 500 meters of volcanoclastic marine sediments, the late Miocene Gatun Formation represents a shallow marine deltaic system draining into the proto-Caribbean sea during the late Miocene (~11-7 Ma). Four samples of felsic tuff of the middle Gatun Formation yield ²³⁸U/²⁰⁶Pb ages for magmatic zircons of 8.24 +/- 0.15 Ma; and two samples give feldspar Ar/Ar plateau ages of ~9-9.8 Ma. These felsic volcanic rocks are likely the product of the earliest El Valle activity. U/Pb and Ar/Ar ages, U-Th/He thermochronology, trace element and isotopic geochemistry as well as sedimentological and stratigraphic field observations constrain a genetic relation between the Late Miocene north Panama Canal basin marine formations and the El Valle volcano. Massive Miocene volcanic activity was responsible for final closure of an important seaway that should have communicated the Caribbean and Pacific waters from about 10 Ma, up to about 4 Ma in central Panama.