Anorthsites are coarse-grained, lightweight volcanic rocks thought to have been formed between 1 and 2 billion years ago. They are mainly composed of 'Plagioclase feldspar' - around 90% CaAl₂Si₂O₈ and 10% NaAlSi₃O₈

They were first dicovered in Northern Labrador, Canada, and have now been identified in many locations - including on the Moon.

The origin of anorthosites is a major unsolved petrogenetic problem in igneous petrology. For most of the numerous types of anorthosite that occur on Earth (and on other planets as well), we cannot agree as to their parental magma compositions, the sources and methods of generation of these magmas, the mechanism of plagioclase accumulation, or their tectonic setting.

Source :Springer Berlin Heidelberg

Further info. Wikipedia