Cumulate Rock - Terminology

Terminology

Cumulates are named according to their dominant mineralogy and the percentage of crystals to their groundmass(Hall, 1996).

• Adcumulates are rocks containing ~100-93% accumulated magmatic crystals in a fine grained groundmass.
• Mesocumulates are rocks with between 93-85% accumulated minerals in a groundmass.
• Orthocumulates are rocks containing between 85-75% accumulated minerals in groundmass.

Cumulate rocks are typically named according to the cumulate minerals in order of abundance, and then cumulate type (adcumulate, mesocumulate, orthocumulate), and then accessory or minor phases. For example:

• a layer with 50% plagioclase, 40% pyroxene, 5% olivine and 5% groundmass (in essence a gabbro) would be termed a plagioclase-pyroxene Adcumulate with accessory olivine.
• a rock consisting of 80% olivine, 5% magnetite and 15% groundmass is an olivine mesocumulate, (in essence a peridotite).

Cumulate terminology is appropriate for use when describing cumulate rocks. In intrusions which have a uniform composition and minimal textural and mineralogical layering or visible crystal accumulations it is inappropriate to describe them according to this convention.