Timscales of rhyodacite formation in Central America: U-series disequilibrium and implications for petrogenetic processes in El Salvador and Nicaragua.

U-series isotopes were measured for rhyodacites from three calderas in El Salvador and Nicaragua along the Central American arc. These data reflect a general trend of increasing (230Th/232Th) from north to south along the arc system. Whole rocks and mineral separates from Ilopongo caldera, El Salvador have (230Th/232Th) of ~ 1.5, and (238U/230Th) of 0.8-1.9. A basaltic enclave from a dome within Ilopongo caldera has a slightly higher (230Th/232Th) of 1.6. Mineral-separate isochrons from this system are consistent with the young age (1600 yrs) of Ilopongo caldera. ^{226}Ra/^{228}Ra were also measured for the Ilopongo samples, and calculated (^{226}Ra/230Th) show a range in values from 0.9- 1.2. To the south of Ilopongo, dacites from Apoyeque caldera, Nicaragua have (230Th/232Th) of 2.5- 2.7, the highest values measured in Central America using plasma mass spectrometry. Rhyodacites from nearby Apoyo caldera, Nicaragua have (230Th/232Th) of ~ 2.2, with the exception of one basalt sample that has (230Th/232Th) of ~ 1.8. These values are consistent with previous measurements for Apoyo caldera. All samples have 230Th excesses, with the exception of one Apoyeque sample that is in equilibrium. In general, volcanic rocks from the Central American arc have ^{143}Nd/^{144}Nd values that are elevated above the mantle array and reflect addition of a high-^{143}Nd/^{144}Nd component from the slab. These new data suggest that this component is also likely to have high (230Th/232Th). Positive correlations between (230Th/232Th) and Ba/Th and also Ce/Pb indicate that this component is likely carried by both fluid and sediment. These data reflect a systematic, southward change in petrogenetic processes operating beneath the Central American arc.