Anorthsites are coarse-grained, lightweight volcanic rocks thought to be 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 composi- tions, 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