A MODEL FOR THE HIGH-TEMPERATURE ORIGIN AND PARADOXICAL DISTRIBUTION OF PEGMATITES IN MAFIC PLUTONS, SMARTVILLE COMPLEX, CALIFORNIA
Pyroxene + hydrous melt = amphibole +/- quartz (1) (Beard et al., 2004)
This is a vapor-absent reaction that can buffer melt water content during crystallization and conceivably preclude water-saturation altogether. In contrast, amphibole and other hydrous phases occur only in trace amounts in the cumulate olivine gabbros. The water-buffering reaction is not encountered during crystallization, the intercumulus melt proceeds to water saturation, and high-temperature plagioclase-clinopyroxene pegmatites form. As crystallization of the pegmatite proceeds, the reaction (1) boundary is eventually reached and incongruent crystallization of amphibole occurs. Nevertheless, once water-saturation occurs, it is irreversible, and is eventually manifested by the low-temperature deuteric alteration associated with the pegmatites. In short, we argue that pegmatite formation in the Smartville gabbros, and likely many other gabbroic plutons, reflects water saturation of a magma at high-temperature. In systems where reaction (1) is attained prior to water saturation, pegmatite formation may be delayed until late in the crystallization history or precluded altogether.