Cumulate rock

Cumulate rocks are the typical product of precipitation of solid crystals from a fractionating magma chamber.

These accumulations typically occur on the floor of the magma chamber, although they are possible on the roofs if anorthite plagioclase is able to float free of a denser mafic melt.

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.

In the above example, the plagioclase and pyroxene need not be pure end-member compositions (anorthite-enstatite), and thus the effect of depletion of elements can be complex.

The foremost problem is that in large ultramafic intrusions, assimilation of wall rocks tends to alter the chemistry of the melt as time progresses, so measuring groundmass compositions may fall short.

Mass balance calculations will show deviations from expected ranges, which may infer assimilation has occurred, but then further chemistry must be embarked upon to quantify these findings.

Though crystallized at high temperature, cumulate can remelt when later intruded by a sill or dyke of magma.

[4] The economic importance of cumulate rocks is best represented by three classes of mineral deposits found in ultramafic to mafic layered intrusions.

However, some anorthosite intrusions contain such pure anorthite concentrations that they are mined for feldspar, for use in refractories, glassmaking, semiconductors and other sundry uses (toothpaste, cosmetics, etc.).

These conditions are created by the high-temperature fractionation of highly magnesian olivine or pyroxene, which causes a relative iron-enrichment in the residual melt.

Sulfide mineral cumulates in layered intrusions are an important source of nickel, copper, platinum group elements and cobalt.

In mafic and ultramafic rocks they form economic nickel, copper and platinum group (PGE) deposits because these elements are chalcophile and are strongly partitioned into the sulfide melt.