MIOCENE TO RECENT VOLCANIC HISTORY OF WESTERN NICARAGUA AND GEOCHEMICAL EVOLUTION OF THE VOLCANIC FRONT

The volcanic record of western Nicaragua documents a significant lull in Central American arc volcanic activity that has persisted from the late Miocene (~ 7 Ma) to the formation of the modern volcanic front around 350 ka. The precise reasons for this lull in volcanic activity are not clear, however it is speculated to have been a consequence of slab rollback.

We report here, geochemical data and 40Ar/39Ar ages on two suites of volcanic rocks that occur between Cosigüina and San Cristóbal, Nicaragua, that appear to fill in some of this missing interval. 1) Encanto, an extinct volcanic center composed mainly of basalts dated between 3.6 to 3.2 Ma. 2) Tinajas, another extinct volcanic center, composed of primarily of basaltic andesite to andesite is dated between 2.5 and 1.3 Ma. In addition, a cluster of isolated hills just south of the Encanto unit is identified as the northwest termination of the Tamarindo Formation that ranges in age from about 14.7 to 11.7 Ma. Geochemistry of the new Tamarindo samples suggests that this formation is geochemically distinct from Coyol volcanism that occurred during the same time period.

Previously reported geochemical analyses of the active and Miocene volcanic fronts show that U/Th values increased by nearly threefold following the “carbonate crash” at 10 Ma. This transition was thought to be abrupt, however new data show that it took place gradually over the last 7 Ma. La/Yb also increases through time and with the distance from the trench, perhaps due to backward migration of the arc towards the trench. Systematic increase in Zr/Nb towards the trench, although indicative of a stronger subduction signal near the trench, shows no correlation with age.

Both the extensive Mid-Miocene Nicaraguan volcanism and the coeval eruption of the Tamarindo and Coyol are consistent with behavior at other circum-Pacific arcs and should not be viewed as anomalous.