| Abundant silicic ash-flow sheets occur in Nicaragua yet little is known about their geochemistry, volume, and age. 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. Two models on the generation of these ignimbrites are evaluated: (1) fractional crystallization of basalt or andesitic melt, and (2) partial melting of previously emplaced arc-related igneous rocks. Chemical and mineralogical variations within and among the seven recognized silicic ash-flow sheets cannot be explained by fractional crystallization alone but rather by partial melting of previously emplaced arc-related igneous rock. This arc-related igneous rock was partially melted by the injection of new mafic magma from the metasomatized mantle wedge. Continued production and ascent of both the silicic and mafic magmas into the magma chamber allowed mingling between the two magmas, and before even reaching compositional and thermal equilibrium, were erupted as ash-flow sheets with distinct chemical variations. REE patterns of the silicic ash-flow sheets mimic the REE trends of the modern arc lavas in Nicaragua indicating a genetic relationship. |
