The Rogaland anorthosites: facts and speculations

The massif-type anorthosites of the province were emplaced as crystal mushes of plagioclase, lubricated by noritic liquids crystallizing along a P-T gradient, and containing aggregates of plagioclase and/or Al-rich orthopyroxene megacrysts, formed in a magma chamber at the base of a thickened crust. Synemplacement deformations were produced in the envelope and within the plutons, where they started in the magmatic stage and ended in the solid stage. The anorthositic suite can be accounted for by three magma types, the composition of which are basaltic, jotunitic and charnockitic. Each magma generates part of the suite, with some overlapping. The basaltic magma is mantle-derived in an undefined geodynamic environment. The alkali to alkali-calcic jotunitic magma is generated as distinct batches with variable crustal signatures, due to contamination by deep-crustal material or direct partial melting of this material. The charnockitic magma can be produced by fractionation of the jotunites but can also result from direct partial melting in granulite facies conditions. For both magma types partial melting is triggered by the hot anorthosite diapirs en route to their final level of emplacement.