Early arc magmatism : Geochemical characteristics of volcanic clasts from Punta Sámara, Costa Rica

ABSTRACT

The geochemical characteristics of volcaniclasts from Punta Sámara, are used to infer magmatic processes that generated Late Cretaceous - Early Paleocene arc lavas. The Ba/La ratios of the volcaniclasts are higher than the ratios for any of the modern lavas from Costa Rica, and the La/Yb ratios are lower; suggesting that the lavas from the primitive arc were generated by greater contribution of slab material than the modern arc lavas. Furthermore, the lower La/Yb ratios imply that the degree of melting was significantly higher and/or that the mantle source was more depleted than in modern Costa Rica. The low Zr/Nb ratios for the modern lavas from central Costa Rica imply a more enriched mantle source (OIB-like), whereas the higher ratios for the primitive arc volcaniclasts imply a more depleted mantle source (MORB-like). We use the geochemical information to infer the conditions of subduction during the early stages of arc magmatism in the southern part of the Central American arc. We infer that the primitive arc was associated with a subducting slab that dip at a steeper angle than the present. The steeper angle would imply that the subducting Farallon plate was older, colder and denser than the present Cocos plate.(AU)