The origin and significance of large, tabular dunite bodies in the Trinity peridotite, northern California

Abstract

Kilometer-sized, tabular dunite bodies are contained within harzburgite, lherzolite and plagioclase lherzolite host rocks in the Trinity peridotite, northern California. An igneous origin for the dunite by crystal fractionation of olivine from a melt is suggested by their tabular shapes, clots of poikilitic clinopyroxene grains, chromite pods, and by analogy to dunite bodies in the Samail and Vourinos ophiolites (Hopson et al. 1981; Harkins et al. 1980; Moores 1969). However, structures and systematic variations in mineralogy and mineral chemistry suggest that at least the marginal few meters of the bodies are residues produced by extraction of a basaltic component from a plagioclase lherzolite protolith. A model is suggested in which a picritic melt ascended through the upper mantle in vertically oriented channels. Part of the dunite in the tabular bodies was produced by fractional crystallization of olivine from the melt. Additional dunite at the margins of the bodies was formed by extraction of a basaltic component from plagioclase lherzolite wall-rocks during partial assimilation by the picritic melt. The latter process is similar to the “wall-rock reaction” discussed by Green and Ringwood (1967) and is essentially zone refining of the the mantle wall rocks by the migrating melt. It is significant because it suggests a mechanism in addition to fractional crystallization for enrichment of incompatible elements in basalts.