gabbro, and norite Leucocratic mafic rocks such as leucotroctolite and leuconorite Though co-eval, these rocks likely represent chemically-independent magmas, likely produced by melting of country rock into which the anorthosites intruded. Importantly, large volumes of ultramafic rocks are not found in association with Proterozoic anorthosites. Physical characteristics Since they are primarily composed of plagioclase feldspar, most of Proterozoic anorthosites appear, in outcrop, to be grey or bluish. Individual plagioclase crystals may be black, white, blue, or grey, and may exhibit an iridescence known as labradorescence on fresh surfaces. The feldspar variety labradorite is commonly present in anorthosites. Mineralogically, labradorite is a compositional term for any calcium-rich plagioclase feldspar containing 50–70 molecular percent anorthite (An 50–70), regardless of whether it shows labradorescence. The mafic mineral in Proterozoic anorthosite may be clinopyroxene, orthopyroxene, olivine, or, more rarely, amphibole. Oxides, such as magnetite or ilmenite, are also common. Most anorthosite plutons are very coarse grained; that is, the individual plagioclase crystals and the accompanying mafic mineral are more than a few centimetres long. Less commonly, plagioclase crystals are megacrystic, or larger than one metre long. However, most Proterozoic anorthosites are deformed, and such large plagioclase crystals have recrystallized to form smaller crystals, leaving only the outline of the larger crystals behind. While many Proterozoic anorthosite plutons appear to have no large-scale relict igneous structures (having instead post-emplacement deformational structures), some do have igneous layering, which may be defined by crystal size, mafic content, or chemical characteristics. Such layering clearly has origins with a rheologically liquid-state magma. Chemical and isotopic characteristics 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