Cumulates and Layered Igneous Rocks

Cumulates form in magmatic intrusions if the crystals and liquid become separated during solidification. This separation is usually gravitationally driven and thus requires low liquid viscosity and a density difference between solids and liquid. Cumulates form due to either removal of the crystals from the remaining bulk magma (e.g. by settling or flotation) or if the liquid remaining in a crystal mush can be removed (e.g. by compaction of the mush) or replaced, permitting continued growth of the accumulated crystals (e.g. by convection). If separation of crystals is complete, an adcumulate results, formed only of minerals grown directly from the bulk magma. If the crystal mush retains abundant interstitial liquid which subsequently solidifies, the result is an orthocumulate. Intrusions in which cumulate formation is extensive are commonly layered on a range of scales. Macro-scale layering is generally a consequence of repeated replenishment of the magma chamber. On a smaller scale, layering commonly has many similarities to that seen in sedimentary rocks, suggestive of significant mass transport in a dynamic system. Other forms of layering are a consequence of self-organisation and recrystallization in a developing crystal mushy layer.