Abstract
Ilopango Caldera was formed ∼1810 years ago by eruption of the Tierra Blanca Joven (TBJ) dacite (70 km3) in central El Salvador. A subsequent eruption in 1880 produced a cluster of dacite domes in the center of Lago Ilopango that contain olive‐bearing enclaves of basaltic andesite. The purpose of this study is to use trace element, isotope, and U‐series data from the TBJ and 1880 eruptions to assess petrogenesis and the timescale of magma storage. We find that although the range of trace element data in the TBJ dacite can be reproduced by simple crystal fractionation of a plagioclase‐ and amphibole‐rich mineral assemblage, the87Sr/86Sr and 207Pb/204Pb data suggest that the 1880 basaltic andesite enclave has a different source than the dacites. This is consistent with U‐series data that show the TBJ dacites have lower (230Th/232Th) than the 1880 basaltic andesite enclave (1.5 versus 1.6, respectively). All Ilopango rocks have 230Th excesses, and the range in (238U/232Th) of the TBJ dacites can be modeled by crystal fractionation of a mineral assemblage including accessory zircon and allanite from a magma that is similar in composition to the enclave. Mineral isochrons yield crystallization ages of <10,000 years, and disequilibrium (226Ra/230Th) values suggest a similarly short residence time. Our data suggest that the large volumes of erupted dacite at Ilopango Caldera are generated very rapidly.
Highlights
Departments of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA [1] Ilopango Caldera was formed $1810 years ago by eruption of the Tierra Blanca Joven (TBJ) dacite (70 km3) in central El Salvador
We find that the range of trace element data in the TBJ dacite can be reproduced by simple crystal fractionation of a plagioclase- and amphibole-rich mineral assemblage, the 87Sr/86Sr and 207Pb/204Pb data suggest that the 1880 basaltic andesite enclave has a different source than the dacites
All Ilopango rocks have 230Th excesses, and the range in (238U/232Th) of the TBJ dacites can be modeled by crystal fractionation of a mineral assemblage including accessory zircon and allanite from a magma that is similar in composition to the enclave
Summary
[2] Ilopango Caldera is located in central El Salvador (N13.6780, W89.0444), 15 km east of the capital city of San Salvador (Figure 1). Deposits from similar calderas such as Aitlán, Ayarza, Coatepeque, Apoyo, Los Chocoyos and Apoyeque have been found in Quaternary deposits throughout Central America, the eastern Pacific Ocean basin [Kutterolf et al, 2008a, 2008b; Sussman, 1985; Vogel et al, 2006; Rose et al, 1999; Mehringer et al, 2005] and the Caribbean seafloor [Jordan et al, 2006; Sigurdsson et al, 2000; Carey and Sigurdsson, 2000]. Despite a large volume of erupted dacite, the minerals do not record a protracted storage time, but instead record generation of the silicic magmas only a short time (
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