The origin of Proterozoic massif-type anorthosites: evidence from interactions between crustal xenoliths and basaltic magma

Plagioclase-rich reaction zones occur around numerous aluminous crustal xen oliths within a suite of Palaeogene sub-volcanic basic sheets on the Isle o f Mull, NW Scotland. The xenoliths consist of a glassy core, containing mul lite needles, generated from the melting of pelitic source rocks. Thick pla gioclase mantles grew at the interface between the aluminous liquid and the enclosing basaltic magma and provide a high-level analogue for the petroge nesis of Proterozoic massif-type anorthosites. Similar interactions between mantle-derived basic magmas ponded at the base of the crust and relatively Al-rich lower crustal lithologies would result in the precipitation of lar ge volumes of plagioclase. Anorthosite massifs were then emplaced at higher crustal levels as crystal-rich mushes within relatively juvenile Proterozo ic crust. The model negates the need to crystallize large volumes of mafic minerals prior to the production of plagioclase-saturated liquids, and also accounts for the significant influence of crustal sources on the isotopic signatures of all members of the anorthosite suite.