Strontium isotope evidence for petrogenesis of Central American andesites

Abstract

THE active volcanic belts of Mexico and of Central America can be considered in two groups (Fig. 1). The Mexican volcanic belt (MVB) forms an irregular line trending away from the Middle America Trench, running from Ceboruco and Colima in the west to San Martin on the Caribbean coast1. In contrast to the MBV, the active Central American volcanic belt runs some 1,200 km from southern Mexico (Tacana) and Guatemala to Costa Rica, broadly parallel with the Middle America Trench1. With the exception of the alkaline volcano San Martin2, these are all calc-alkaline andesite volcanoes1. Between Mexico and El Salvador, the volcanic chain is built on a crust which includes Precambrian and Lower Palaeozoic crystalline rocks3,4. Southeast of El Salvador, in Nicaragua and Costa Rica the crust is composed mainly of Cainozoic volcanic and sedimentary rocks resting on Cretaceous ultramafic complexes3–6. This change in basement character does not seem to be reflected in the composition of volcanic rocks7. Here we report new Sr isotope data for Cainozoic volcanic rocks from Costa Rica, and by comparing these with earlier published data we conclude that all of these volcanic rocks have their origin in the mantle and that a sialic contribution to their origin is insignificant.