GRAVITY AND GEOCHEMICAL CONSTRAINTS ON THE EVOLUTION OF THE EL VALLE VOLCANO, PANAMA

El Valle is the eastern most 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 Ma. Both the old and young groups are calc-alkaline in nature. However, the old group is dominantly andesitic, has flat REE curves, low Sr/Y ratios and has a plagioclase and pyroxene mineralogy. In contrast, the younger group is dominantly dacitic, hornblende bearing, has low HREE values, high Sr/Y ratios and 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 characteristics.

El Valle is also characterized by a large 25-30 mgal negative Bouguer Gravity anomaly that is apparent in both surficial and satellite gravity measurements. Initial surficial data indicates Bouguer anomalies vary from 32 mgal at the edge of the volcano to -5 mgal in its central caldera. Field lithologic observations paired with the gravity measurements suggest that the negative gravity anomaly is caused by low-density young group dacitic pyroclastic rocks and the most recently erupted El Hato ignimbrite, both of which exhibit the adakitic signature. Quantitative modeling of the gravity data will allow the volume of the young group volcanic rocks to be determined, which can be fed into petrologic models to determine the overall magmatic flux at the El Valle volcano. This in turn will help 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.