VARIATIONS IN LAVA COMPOSITION ASSOCIATED WITH FLOW OF ASTHENOSPHERE BENEATH SOUTHERN CENTRAL-AMERICA

Existing trace element and isotopic data, as well as new U-series data for Nicaragua and Costa Rica, are examined in light of the recent sug gestion that asthenospheric mantle is flowing northward behind the sub ducting Nazca plate around the northwest corner of South America. Lava s from central Nicaragua have trace element and isotopic compositions suggesting that they are derived from relatively depleted mantle with a long history of interaction with the subducting Cocos plate. Lavas f rom northern and central Costa Rica have compositions indicating a mor e enriched mantle source that has been influenced little by the subduc ting slab. These compositional traits vary irregularly between the two regions, suggesting that mantle compositions and subduction influence s intermingle in a complex manner beneath northern Costa Rica. The dip of the subducting slab decreases abruptly southward, and accretion of sediment begins in this same vicinity. These changes in composition a nd physical parameters may all be induced or influenced by an influx o f asthenospheric material from behind the retreating Nazca plate.