Revista del Centro de Investiagaciones Geoscientificas, Nicaragua

Tertiary ignimbrites are an important volcanic formation in Central America, extending from the Mexican-Guatemalan border to southern Nicaragua. The immensity and extent of these large-volume, silicic magmas presents an important problem in igneous petrogenesis. Tephra layers recovered in deep-sea cores collected during ODP leg 165 indicates that there were two major ignimbrite flare-ups in the region, one at 32 to 35 Ma; the other in early Miocene between 15 and 20 Ma. This study focuses on the Miocene marine tephra layers and glassy samples from the Central American Ignimbrites. A total of 171 glass samples and selected minerals were analyzed by electron-microprobe for major oxides and laser-ICPMS for trace elements. The ignimbrite magmas show a limited range in major oxide composition, with all but 10 being rhyolitic. Cluster analysis, based on REE, indicates that the ignimbrites can be separated into nine distinct groups. Most of the ignimbrites have light-REE levels of enrichment two to ten times that of E-MORB and heavy-REE values close to those of E-MORB. These REE patterns are close to those of continental crust. In addition, each group shows distinct enrichments and depletions of certain elements. For example: Europium anomalies show a wide range between groups, from -3.45 to 2.43, while three of the geochemical groups have no Eu anomaly. Some geochemical groups, with moderate LREE enrichment, minor middle-REE depletion, and no HREE depletion or enrichment relative to E-MORB, are virtually identical to bulk continental crust. Other geochemical groups, with large negative Eu anomalies, large LREE, and moderate HREE enrichment relative to E-MORB, have features suggesting an assimilation-fractional crystallization process. We will utilize the geochemical data to evaluate the various petrogenetic processes that may have contributed to the origin of these large-volume, silicic magmas.