Gravity and Geochemical Constraints on the Structure and Evolution of the El Valle Volcano, Panam?

El Valle is the easternmost volcano in the Central American Volcanic Arc, and is characterized by two periods of volcanic activity, an Old Group from 10-5 Ma and a Young Group from 3-0.03 Ma. Both the Old and Young Group's are calc-alkaline in nature. However, the Old Group is dominantly andesitic, has flat REE patterns, low Sr-Y ratios and contains plagioclase and pyroxene phenocrysts. A subgroup of the older samples also contains biotite. In contrast, the younger group is dominantly dacitic, hornblende-bearing, has low HREE content, high Sr/Y ratios and overall exhibits adakitic characteristics. The transition between the two geochemical groups occurs gradually between 10-3 Ma, at which point the adakitic signature is fully developed. The modern El Valle volcanic edifice is dominated by voluminous Young Group eruptive products (e.g. the El Hato Ignimbrite) with adakitic-like characteristics.;El Valle is also characterized by a large 40-50 mGal negative Bouguer Gravity anomaly that is apparent in both surficial and satellite gravity measurements. Initial surficial data indicates Bouguer anomalies vary from 40 mGal at the edge of the volcano to --10 mGal in its central caldera. Field lithologic observations paired with the gravity measurements and modeling suggest that the negative gravity anomaly is primarily due to a low-density batholith beneath the volcano.;Samples collected during the gravity survey where geochemically analyzed to distinguish between the various proposed petrogenetic/tectonic models in southern Panama (e.g. the existence of a slab window, oblique subduction, slab-melting or subduction erosion of Galapagos material). One significant quandary is that geochemical evidence suggests the existence of subduction whereas geophysical data shows a lack of a pronounced Benioff Zone. Modeling suggest that the geochemical signatures can be reproduced via the partial melting of Cretaceous-Early Tertiary lower crustal arc rocks.