285-12 The Evidence is in the Rocks: Why Massif Anorthosite-Ferroan Granite Suites Did Not Form in Arcs

Abstract:

The AMCG (anorthosite-mangerite-charnockite-granite) suite of massif anorthosite, monzonite, and ferroan granite occurs almost exclusively in the Proterozoic, and Phanerozoic analogs are extremely rare. Accordingly, the tectonic environment in which they occur is not obvious, and some authors have proposed an origin by arc magmatism above subduction zones. However, petrologic evidence in the rocks themselves precludes this origin. The composition of the likely parent magma to anorthosite, high-Al gabbro, is unlike basalt from MORB, oceanic plateaus, or subduction zones. AMCG high-Al gabbro most closely matches basalt from the Galapagos plume. Experimental phase equilibria studies suggest that melting took place at 10-12 kbar, shallower than arc melting. The mafic rocks in the AMCG suite are compositionally distinct from those in arcs: AMCG gabbros have olivine with Fo₆₅ or less whereas arc gabbros have more magnesian olivine (<Fo₇₀). AMCG gabbros have plagioclase with An₆₅ or less, whereas arc gabbros have relatively calcic plagioclase (>An₇₅).

Ferroan granites of the AMCG suite are alkalic to alkali-calcic, unlike magnesian, calcic to calc-alkalic arc magmas. Ferroan granites are relatively reducing, crystallizing at or below FMQ, whereas arc magmas tend to be relatively oxidizing, having crystallized at oxygen fugacities 1-2 log units above FMQ. Ferroan granites have lower water contents and crystallize at higher temperatures than do arc magmas. Where ferroan granitic magmas intrude crust with isotopic compositions distinct from contemporary mantle, it is clear that these magmas have a dominantly mantle origin.

The long-lived Nuna-Rodinia supercontinent persisted throughout the Proterozoic and may have insulated the underlying mantle and caused large areas of mantle upwelling that produced the AMCG magmatic suites. If instead the origin of the AMCG suite were related to an arc setting, it must have involved large amounts of magma not at all typical of known arc magmas. Impossible? No. Implausible? Yes!

© Copyright 2026 The Geological Society of America (GSA), all rights reserved.