The formation of massif anorthosite: Petrology in reverse

Massif anorthosites form when basaltic magma differentiates in crustal magma chambers to form low-density plagioclase and a residual liquid whose density was greater than that of enclosing crustal rocks. The plagioclase and minor pyroxene crystallized in-situ on the floor of the magma chamber to produce the anorthosite complex, and the residual liquid migrated downwards, eventually to solidify as dense Fe-rich cumulates some of which were removed to the mantle. These movements were facilitated by high temperatures in Proterozoic continental crust, thus explaining the restriction of large anorthosite massifs to this period in Earth history.

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► The fate of residual liquid left after plagioclase accumulates in massif anorthosites is unknown.
► This liquid is denser than crustal rock and migrated downwards into the lower crust.
► The predominance of massif anorthosites in the Proterozoic may be related to the thermal state of continental crust.