Miocene zircon crystals in dacite from Ilopango Caldera, El Salvador: Evidence for recycling of plutonic rocks

Ilopango Caldera is located in El Salvador and is part of the Central American Volcanic Arc (CAVA) that extends from southern Mexico to Panama. The volcanic arc is situated on crust that ranges in age from 150-28 Ma and is covered with Miocene-Recent volcanic ash and pyroclastic flow deposits. Several large eruptions are associated with Ilopango Caldera, the most recent are from the Tierra Blanca Joven (TBJ) eruption, which produced massive pyroclastic flows 1600 years BP. Older eruptions from Ilopango Caldera are referred to as the Tierra Blanca (TB) deposits, and the TB2 ignimbrite has been dated at 12,000 years. The objective of this research is to use the ages of zircon crystals from the TBJ and TB2 eruptions to establish and compare storage times for these magma reservoirs. We used a CAMECA ims 1270 at UCLA's NSF National Ion Microprobe Facility in order to obtain U-Pb and U-Th ages for individual zircon crystals from each eruption. Depth profiling and U-Pb analyses were performed on both of the zircon crystals using established analysis techniques. The data show that zircon from both eruptions have 15 Ma old cores with thin rims (few μm) that are consistent with the young eruption ages. In both cases, however the transition from core to rim composition is abrupt and does not record continuous crystallization of the zircon crystals. We conclude that the presence of the old cores is consistent with assimilation of middle Miocene plutonic rock by juvenile magma during Quaternary activity of Ilopango Caldera. The most likely source of 15 Ma old zircon are the plutons associated with middle Miocene explosive volcanism in Central America. Ash deposits recovered from the sea floor (via ODP studies) record extensive explosive volcanism from 13-15 Ma that can be traced to ignimbrite deposits of the Chalatenango Formation in south-central Guatemala and El Salvador. We conclude that 1) the zircon crystals record only brief pre-eruptive crystallization histories for the TBJ and TB2 eruptions of Ilopango Caldera, 2) that the presence of nearly identical crystal ages in the TBJ and TB-2 rocks mean that the same plutonic source has been recycled since the early activity underneath Ilopango Caldera and 3) that the assimilant is likely the plutons related to the Miocene paleo-arc in El Salvador.