Regional AI-Fe mafic magmas associated with anorthosite-bearing terranes

Abstract

PROTEROZOIC anorthosite massifs are nearly monomineralic accumulations of plagioclase feldspar, which must result from unusual conditions of crystallization and emplacement1,2. Their chemistry is compatible with derivation from a high-alumina-basalt parent of mantle origin1, and mafic magmas rich in aluminium and iron have long been suspected of having an important connection to anorthosites and their parent magmas1–4. Regional examples of mafic magmas clearly compatible with an anorthosite lineage have not been generally recognized3, in part because of the paucity of candidates within the anorthosite massifs themselves. Samples of such magmas should also be sought in the country rocks surrounding the massifs. Here we report the widespread occurrence of Al–Fe mafic magmas in the anorthosite-bearing Adirondack Highlands of New York state. They are chemically distinct from typical high-AI basalt, Fe basalt and most common magma types. Identification of similar Al–Fe basaltic magmas in other areas suggests that such magmas are characteristic of anorthosite-bearing terranes.