A MODEL FOR THE HIGH-TEMPERATURE ORIGIN AND PARADOXICAL DISTRIBUTION OF PEGMATITES IN MAFIC PLUTONS, SMARTVILLE COMPLEX, CALIFORNIA

The occurrence of pegmatitic rocks in the zoned gabbro-diorite plutons of the late Jurassic Smartville Complex is paradoxical. While pegmatitic gabbros are common in the mafic cumulate olivine gabbros within the zoned plutons, they are virtually absent from the more evolved rock types. We argue that this paradox is resolved by examining the crystallization/temperature history of the rocks in question. The evolved rocks in the plutons consist of 2-pyroxene hornblende biotite gabbro and diorite. The amphibole in these rocks occurs as a partial replacement of pyroxene that forms as a consequence of the incongruent crystallization reaction:

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.