PETROLOGY OF TERTIARY IGNIMBRITE PROVINCE IN CENTRAL AMERICA

The Tertiary Ignimbrite Province of Central America is one of the largest regions of silicic volcanism on Earth. Two intense periods of explosive volcanism, or ignimbrite flare-ups, have been identified on the basis of biostratigraphic dating of tephra layers in Caribbean Sea sediments during ODP Leg 165. The first ignimbrite flare-up occurred during the Eocene and was followed by a period of quiescence that ended at the beginning of the Miocene. The mid-Miocene event lasted approximately 10 m.y. and constitutes the majority of the deposits found in the ignimbrite province of Honduras and Nicaragua today. The origin of these large-volume silicic magmas remains a fundamental problem, as does the conditions of magma source region. We have studied the petrology of Central American ignimbrites, using fresh basal vitrophyre and obsidian clasts for this research. Electron microprobe analysis has been performed on matrix glass and coexisting minerals for 29 samples of Tertiary ignimbrites from Honduras and Nicaragua. All of the ignimbrites are rhyolitic to alkali-rhyolitic, with the exception of one trachydacite. Coexisting phenocrystic minerals of plagioclase and iron-titanium oxides are found in all samples. In general, there are two populations of ignimbrites based on the content of secondary phenocrysts. The first population (>50%) typically contains coexisting orthopyroxene and clinopyroxene, although in some instances opx is absent. The second population includes amphiboles and biotites, occasionally with orthopyroxenes. In several samples, quartz, sanidine, anorthoclase, and apatite occur in trace amounts. The mineral and matrix glass chemistry has been used to infer conditions of the magma chamber, also known as intensive parameters (temperature, pressure, oxygen fugacity, and PH2O), through a variety of methods.