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. High-alumina orthopyroxene megacrysts (HAOMs) Many Proterozoic-age anorthosites contain large crystals of orthopyroxene with distinctive compositions. These are the so-called high-alumina orthopyroxene megacrysts (HAOM). HAOM are distinctive because 1) they contain higher amounts of Al than typically seen in orthopyroxenes; 2) they are cut by numerous thin lathes of plagioclase, which may represent exsolution lamellae; and 3) they appear to be older than the anorthosites in which they are found. The origins of HAOMs are debated. One possible model suggests that, during anorthosite formation, a mantle-derived melt (or partially-crystalline mush) was injected into the lower crust and began crystallizing. HAOMs would have crystallized out during this time, perhaps as long as 80–120 million years. The HAOM-bearing melt could then have risen to the upper crust. This model is supported by the fact that aluminum is more soluble in orthopyroxene at high pressure. In this model, the HAOM represent lower-crustal cumulates that are related to the anorthosite source-magma. One problem with this model is that it requires the anorthosite source-magma to sit in the low crust for a considerable time. To solve this, some authors suggest that the HAOMs may have formed in the lower crust independent of the anorthosite source-magma. Later, the anorthosite source-magma may have entrained pieces of the HAOM-bearing lower crust on its way upward. Other researchers consider the chemical compositions of the HAOM to be the product of rapid crystallization at moderate or low pressures, eliminating the need for a lower-crustal origin altogether. Origins of Proterozoic anorthosites The origins of Proterozoic anorthosites have been a subject of theoretical debate for many decades. A brief synopsis of this problem is as follows: The problem begins with the generation of magma, the necessary precursor of any igneous rock. Magma generated by small amounts of partial melting of the mantle is generally of basaltic composition. Under normal conditions, the composition of basaltic magma requires it to crystallize between 50 and 70% plagioclase, with the bulk of the remainder of the magma crystallizing as mafic minerals. However, anorthosites are defined by a high plagioclase content (90–100% plagioclase), and are