Origin of the Silicic Ash-flow Sheets in Southern Nicaragua

Abundant silicic ash-flow sheets occur in southern Nicaragua yet little is known about their distribution, age, and geochemistry. Their origin is a matter of considerable interest because the generation of silicic magma is generally attributed to the assimilation and/or melting of an underlying continental crust. However, no continental crust occurs in this area. This southern part of the Central American Volcanic Arc is located in the Chorotega block, which is composed of a thickened oceanic crust (Hauff et al., 2000). This is in contrast with the northern section of the arc, the Chortis block, which is underlain by continental crust. Here, we describe the occurrence and composition of silicic ash-flow sheets in southern Nicaragua. Seven ash-flow sheets were identified based on their chemical characteristics. These ash-flow units are Apoyo, Las Maderas, Las Sierras, and 4 informally named units referred here as: Coyol, Monte Galan, Ostocal, and San Rafael, all belonging to the Coyol Formation. The age of the Apoyo unit is 23,000 (Sussman, 1982) whereas the other ash-flow units maybe as old as Miocene (Ehrenborg, 1996). The composition of pumice fragments from these sheets range from 50.9 to 71.5 wt. % SiO₂. The most frequent occurring compositions (mode) in six of these units ranges from 65 to 70 wt. % SiO₂, whereas the Las Maderas unit has a mode at 63% SiO₂. Plots of Rb, Sr, Zr, total alkalies, Fe₂O_3(t), MgO, and K₂O versus SiO₂ for each of these ash-flow sheets define distinct chemical trends. REE spider plots are relatively flat. Some ash-flow sheets (Ostocal, Las Maderas and Las Sierras) contain a small negative Eu anomaly (0.77 to 0 .90), whereas the Apoyo contains a slight positive Eu anomaly (1.09 to 1.13). Chemical variations among these sheets cannot be related by fractional crystallization. Overall the trace element variations (e.g. U/Th, Ba/Nb ratios) of these silicic sheets mimic the variation in the trace element variations in the modern arc, which to us, means there is a genetic relationship between the modern arc and the silicic magmatism. Because it is difficult to generate large-volume silicic magmatism by fractional crystallization of mantle melts, we are evaluating models where silicic magmas are produced by partial melting of previously emplaced arc-related igneous rocks.