Anorthosite

Proterozoic anorthosites are typically >90% plagioclase, and the plagioclase composition is commonly between An40 and An60 (40–60% anorthite). This compositional range is intermediate, and is one of the characteristics which distinguish Proterozoic anorthosites from Archean anorthosites (which are typically >An80). Proterozoic anorthosites often have significant mafic components in addition to plagioclase. These phases can include olivine, pyroxene, Fe-Ti oxides, and/or apatite. Mafic minerals in Proterozoic anorthosites have a wide range of composition, but are not generally highly magnesian. The trace-element chemistry of Proterozoic anorthosites, and the associated rock types, has been examined in some detail by researchers with the aim of arriving at a plausible genetic theory. However, there is still little agreement on just what the results mean for anorthosite genesis; see the 'Origins' section below. A very short list of results, including results for rocks thought to be related to Proterozoic anorthosites, Some research has focused on neodymium (Nd) and strontium (Sr) isotopic determinations for anorthosites, particularly for anorthosites of the Nain Plutonic Suite (NPS). Such isotopic determinations are of use in gauging the viability of prospective sources for magmas that gave rise to anorthosites. Some results are detailed below in the 'Origins' section.