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

Abstract

Bulk analyses of plagioclase and high-alumina orthopyroxene megacrysts (decimetreto 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 similar Mg-numbers, but different wollastonite, Sr, Mn and Cr contents. A Rayleigh fractional crystallization model of the trace element concentrations vs MgO content in orthopyroxene constrains the cotectic orthopyroxene:plagioclase proportions to 0·33:0·67 in both magma types before the appearance of Fe^Ti oxide minerals on the liquidus at F 0·7 (where F is melt fraction). Polybaric crystallization is recorded by variable alumina contents in the orthopyroxene megacrysts (2·3^8·5 wt %), corresponding to crystallization pressures in the 3^13 kbar range as shown by experimental data. This implies that the high-alumina orthopyroxene megacrysts mainly crystallized en route during diapiric rise of the anorthositic mush. Modelling the plagioclase compositions with experimentally determined partition coefficients for Ca, K, Sr and Ba confirms that pressure variation during polybaric crystallization is the main controlling factor on compositional variations in the anorthosite pluton.