Abstract
Gabbro pegmatites are common in cumulate olivine gabbros in the Smartville complex, northern California. The pegmatites occur as pods and segregations and consist primarily of calcic bytownite or anorthite and clinopyroxene, with minor orthopyroxene and olivine. Amphibole is common both as a late magmatic phase and as a subsolidus alteration of magmatic amphibole and pyroxene. Many of the pegmatites are zoned, with anorthositic rims and pyroxenitic cores. Fine-grained, idiomorphic granular gabbro in some pegmatite cores may be analogous to the quench aplites reported from many granitic pegmatites.
The igneous mineralogy and the mineral chemistry of five pegmatite pods are indistinguishable from that of their host gabbros. This suggests that these pegmatites formed in place and in equilibrium with their gabbro host. One pegmatite intrusive dike is more mafic than its host gabbro, suggesting that this pegmatite melt did not form in place. The presence of gabbro “aplites,” the zoning of the pegmatites, and the high temperatures calculated from coexisting pyroxenes support the idea that these pegmatites formed by crystallization of a melt rather than by subsolidus replacement. High fluid pressures are suggested by the compositions of coexisting olivine and plagioclase, by the presence of late magmatic amphibole, by the presence of fluid inclusions, and by the restriction of subsolidus hydrous alteration assemblages largely to the cores of the gabbro pegmatites.
We propose that the pegmatites formed from a mafic intercumulus melt in the presence of a fluid phase. The evolution of the fluid phase may have been enhanced by the drop in confining pressure that accompanied re-emplacement of the cumulate gabbros as crystal mushes. The disruption of the cumulate pile may have enhanced the migration and accumulation of the intercumulus melt.
