Anorthotization

Anorthotization is a geological process that describes the enrichment of plagioclase feldspar in igneous rocks, particularly mafic and ultramafic magmas. It occurs when more calcic plagioclase crystals, which are denser, settle out of a cooling magma. As these crystals precipitate and accumulate, the remaining liquid magma becomes progressively enriched in the more sodic components of plagioclase. This residual liquid then crystallizes to form rocks with a higher proportion of sodic plagioclase.

The process is typically observed in layered intrusions, such as the Bushveld Complex in South Africa or the Skaergaard intrusion in Greenland. These intrusions are characterized by distinct layers of cumulate rocks, where mineral phases have settled gravitationally. In the case of anorthotization, layers rich in calcic plagioclase (anorthosites) can form at the base of these intrusions, while overlying layers may show an upward trend towards more sodic plagioclase compositions.

Anorthotization is a significant factor in the differentiation of magmas and can lead to the formation of economically important ore deposits. For instance, the enrichment of certain trace elements can be associated with the anorthositic layers formed during this process. Understanding anorthotization helps geologists interpret the evolution of magma chambers and the genesis of various rock types and associated mineralization.