LINKING THOLEIITES AND SILICA-UNDERSATURATED ALKALIC ROCKS: AN EXPERIMENTAL STUDY

The silica-undersaturated magma series in anorogenic regimes have been attributed to either low pressure fractionation of a silica-undersaturated starting composition or from varying amounts of partial melting of a garnet lherzolite. Therefore, the primary silica-undersaturated alkalic characteristics are generally considered to be inherited from the parental melts. However, alkalic magmas commonly coexist temporally and spatially with tholeiitic magma and any possible relationship between these magma types has never been fully understood.

In order to explore a possible compositional link between silica-undersaturated and tholeiitic magmas, fractional crystallization experiments were conducted on a continental flood basalt (olivine tholeiite) from the Snake River Plain. These experiments were carried out in graphite capsules in piston cylinder apparatus at 14.3 kbar with 0.05, 0.35, and 0.5 wt% water. Under these conditions, evolving liquids became both alkalic and silica-undersaturated. Under the drier conditions, melts become more ne-normative and total alkali-enriched at higher temperatures than the experiments with higher water contents. Dry (0.05 wt% water) experiments crystallize the sequence clinopyroxene and then clinopyroxene + plagioclase and produced liquids similar to some liquids along the Tristan Da Cunha alkalic series. Higher water content (0.35 and 0.5 wt% water) experiments crystallize in the sequence clinopyroxene, clinopyroxene + plagioclase, and finally clinopyroxene + plagioclase + orthopyroxene + spinel and produce liquids that extend along the Gough Island alkalic series.

These experiments suggest that the major element characteristics of alkalic silica-undersaturated suites could arise from fractionation of tholeiitic magma, rather than requiring an alkalic, silica-undersaturated parental liquid.