Anorthosites in the Early Crust of the Earth and on the Moon

Abstract

THE recent report from Houston¹ proposed that the lunar highlands may be composed “predominantly” of anorthosite and gabbroic anorthosite. O'Hara has therefore suggested that some of the Pre-Cambrian anorthosites may be reworked slices of the Earth's primordial crust. The aim of this report is to present evidence for such a relationship between lunar and early terrestrial anorthosites. Apart from the suggestion by Olsen², this has not been considered in the literature on either the lunar samples or on the evolution of the Earth–Moon system. In their comparisons of the lunar anorthosites with terrestrial rocks, Wood et al.³ have followed the commonly accepted idea that there are only two varieties of anorthosites—the massive Adirondack and the stratiform Bushveld types. There is also a third type, however (first recognized as such by Harpum⁴ and re-emphasized by Bridgwater⁵); a layered calcic anorthosite found in some high-grade basement terrains. Anorthosites in this third group are mostly the oldest plutonic rocks in the crust with any decipherable mode of origin, having been formed more than 3,000 m.y. ago and possibly as old as 3,500 m.y., and they are extremely similar chemically to the lunar anorthosites. Indeed, the similarities are enough to suggest that these are the first terrestrial rocks to have any marked affinities with those on the Moon.