CONTINUITY (BA) AND CHANGE (U) IN CENTRAL AMERICAN GEOCHEMISTRY: NEW EVIDENCE FROM THE MIOCENE BALSAMO FORMATION IN EL SALVADOR

New data from Balsamo Formation lavas (Miocene) in El Salvador double the regional geochemical coverage for the Miocene arc and increase our understanding of the temporal geochemical evolution of Central American volcanics. Our results for El Salvador support and extend those of Plank et al., (2002) for Nicaragua. Across El Salvador, the Miocene lavas show a decrease in Ba/La from SE to NW; similar to the gradient found in modern lavas. In contrast, U/Th is low and constant across El Salvador, clearly lacking the gradient found in modern lavas. The similar distribution of Ba/La ratios in both Miocene and modern arcs indicates that the incoming mass of Ba and its delivery process has not varied over this time interval. In contrast, the Miocene arc is depleted in U relative to the modern arc. The temporal variation of U in lavas follows a similar variation in Cocos Plate sediments. Prior to 10 my, the Cocos sediments were carbonates with little U; after 10 my, the sediments deposited were hemipelagic muds rich in organic material and U. The high U/Th values of the modern arc are the result of the cycling of this younger, U-rich sediment into the volcanoes.

A few of the Salvadoran samples have intermediate U/Th ratios. The ages of these samples are not yet known, but they appear to mark the transition between magmas generated with input from just the older carbonate sediments and magmas generated with input from both the carbonates and the younger U-rich hemipelagic sediments. Further sampling in this area, coupled with age dating, should define the time when the U-rich hemipelagic sediments reached the zone of magma genesis beneath the volcanic front.