Troctolite

Troctolite is a coarse-grained ultramafic igneous rock in which olivine and plagioclase are the dominant minerals, with little to no pyroxene. The mineral assemblage typically comprises abundant olivine (Fo-rich) and calcium-rich plagioclase (anorthite to labradorite), with only minor interstitial pyroxene, chromite, or spinel in some samples. The name troctolite reflects its balanced olivine–plagioclase composition, a texture characteristic of cumulate rocks formed during crystallization of a magma.

Troctolites form mainly as cumulate rocks within layered mafic–ultramafic intrusions or in ophiolitic complexes. As a basaltic to ultrabasic melt crystallizes, early-formed crystals of olivine and plagioclase settle out and accumulate to create coarse-grained layers enriched in these minerals. The exact proportions vary, and some rocks may contain trace pyroxene or accessory minerals such as magnetite or chromite.

In petrology, troctolite is part of the olivine–plagioclase cumulate assemblage and relates to other cumulate rocks such as dunite (olivine-dominated) and harzburgite (olivine plus orthopyroxene). Troctolites provide information about crystallization sequences in basaltic magmas and contribute to understanding the composition of the mantle and crustal sections in ophiolites.

See also: dunite, harzburgite, cumulate rock, ophiolite.