Fig. 6. Proposed model for the formation of the Marcy and Morin massif-type anorthosites.

Isotope systematics and bulk compositions of anorthosites and associated mafic rocks point to melting of slab-top LTAOC that had dehydrated to various degrees before melting. Upwelling asthenosphere provides a heat source to melt the slab and likely melted to provide additional mafic magma. Texturally early pyroxene megacrysts do not show δ¹¹B values lower than those of unaltered mantle, suggesting that early slab-derived fluids and sediment melts also contributed to their formation. Varied δ¹¹B signals within one rock likely record changes to magma sources from different slab depths through time (28) or mixed cumulate populations. Oceanic crust cross section is after (43).