Evaporites as precursors of massif anorthosite

There is evidence that evaporites were present in premetamorphic rocks associated with massif-type anorthosite. In the Grenville Series, which contains the Adirondack anorthosite massifs, anhydrite and halite are preserved in marble. Evaporites and their enclosing shales represent concentrations of Na, Ca, Al, and Si- the elements essential to formation of anorthosite. Permissive evidence for derivation of anorthosite from evaporite sequences includes the high oxidation state of some massif anorthosite, consistent with derivation from oxidized metasedimentary rock; widespread occurrence of scapolite in anorthosite and other Grenville rocks; Sr-isotope data, which yield ⁸⁷ Sr/⁸⁶ Sr ratios that correspond to the values for Precambrian sea water; and O-isotope data, which yield metasedimentary ¹⁸O/¹⁶O ratios for the Adirondack anorthosites.

A chemical model is proposed whereby fluids generated by dissolution of evaporite under high-grade metamorphism react with pelitic rock. The initial high ratios of a_Na⁺/a_H⁺ and a_Ca²⁺/a_H⁺ maintain the plagioclase stability field. As the fluid migrates and reacts, the Na and Ca concentrations are diminished, and the release of H⁺ to the fluid is buffered by sulfate and/or carbonate, but the concentration of K⁺ increases. When the ratio of a_K⁺/a_H⁺ is sufficiently high, both plagioclase and K-feldspar are stable, and gradations of anorthosite into syenite, quartz syenite, or charnockite are produced. The fluid may have a composition intermediate between a supercritical aqueous solution and silicate magma and may evolve toward a low-melting composition in the quartz-plagioclase-orthoclase system. Anatexis could account for potassic dikes that sometimes cut anorthosite borders.

Textural features of massif anorthosite, the replacement origin of anorthosite at the borders of some massifs, widespread reports of metasomatism in Grenville rocks, and well-documented reports of other metasomatic anorthosites support the model.

Massif anorthosites are restricted to an age range of 1.0 to 1.7 b.y. because (1) sea water was not sufficiently saline to produce evaporites during earlier Earth history and (2) Paleozoic and younger shelf-type evaporites are not yet metamorphosed, but remain on the stable cratons.