The Ana-Sira anorthosite massif, part of the Pre-Cambrian anorthositic Egersund complex, is predominantly of practically unzoned andesine feldspars with average 45% An (range 39-50 %) and 3% Orisomorph (1-5 %). Sporadically occurring Iabradorescent plagioclases contain 51 % An and 2.5 % Orisomorph on an average. Probably the regional distribution of these values throughout the massif is not entirely irregular. The plagioclases in the anorthosite and the leuconoritic parts are divided into different types on the basis of: their macroscopic and microscopic features (size, form, inclusions, antiperthitic exsolution, twinning etc); their chemical compositions; and their structural states. They represent various p, T-conditions during the magmatic and later history. After the magmatic phase, presumably with meso- and adcumulation, the determinant factors were deformation, granulation, and recrystallization. Three different main ore bodies (ilmenite norites) are enclosed in the anorthosite. In general, their plagioclases show almost the same An range as the anorthosite andesines, but they seem to have somewhat different trace and minor element contents, and mostly they are less ordered than the latter.
Plagioklase im antorthosite-komplex von Åna-Sira, SW-Norwegen. Petrologische und chemische untersuchungen