Polybaric Fractional Crystallization of High-alumina Basalt Parental Magmas in the Egersund^Ogna Massif-type Anorthosite (Rogaland, SW Norway) Constrained by

Bulk analyses of plagioclase and high-alumina orthopyroxene mega-crysts (decimetre- to metre-sized crystals with plagioclase lamellae) from the late Proterozoic Egersund^Ogna massif-type anorthosite (Rogaland anorthosite province, SWNorway) are used to constrain the parental magma compositions and differentiation processes within this 30 km diameter diapir. Spatial compositional variations show that two types of anorthosite occur: high-Sr (720^1090 ppm) andesine anorthosite (An48 4) in the centre of the intrusion and low-Sr (320^620 ppm) labradorite anorthosite (An59 6) in the margin. Two populations of orthopyroxene megacrysts are clearly discriminated by their Mn and Cr contents, but display similar ranges of Mg-number (55^79). Interpretation of trace element data and comparison with inversion models suggest that the two types of anorthosite result from crystallization of different parental magma compositions with high-alumina basalt affinities and simila