Chapter 6 Proterozoic Anorthosite Complexes

Abstract

Anorthosite massifs (up to 1000s of km 2 in area) appear to represent a unique magmatic episode in the mid-Proterozoic. They have many characteristics that distinguish them from intrusive complexes emplaced either in the Archean or Phanerozoic. Of primary importance is the dominance of massive to weakly layered anorthositic plutons containing between 75% and 95% plagioclase of intermediate composition. Within these plutons, orthopyroxene or olivine with intermediate compositions are the most common mafic minerals. The anorthositic rocks are characteristically very coarse-grained with unzoned or weakly zoned subhedral plagioclase commonly between 1 and 10 cm and rarely up to 1 m in length. Although many complexes contain large volumes of associated granitic rocks, mafic rocks (including layered gabbroic intrusions) rarely comprise more than 10%. Some of the associated mafic rocks in most massif anorthosite complexes are Fe- and Ti-rich diorites or jotunites; Fe-Ti oxide deposits are common in many complexes. Ultramafic rocks are essentially absent except for the small isolated bodies of oxide-rich cumulates associated with mafic rocks.