Large volumes of anatectic melt retained in granulite facies migmatites: An injection complex in northern Quebec

The Opinaca Subprovince in the Superior Province of northern Québec is a metasedimentary belt characterized by metagreywacke rocks that record a late Archean granulite facies metamorphism and contains abundant thin veins of leucogranite. Regional-scale study of this subprovince provides insight to the net budget of anatectic melt (loss versus accumulation) in the lower middle continental crust during anatexis.A petrological study indicates that the metagreywackes partially melted at ~ 820 °C and ~ 7 kbar and generated < 10% melt with orthopyroxene ± garnet as peritectic phases. The metagreywackes contain various amounts of leucogranite in thin veins, sills and dykes injected sub-parallel to the subvertical E–W striking main foliation. Study of more than 1070 outcrops indicates that the “typical outcrop” contains ~ 63% of leucogranite; far more than the maximum 10% of melt produced in-situ from the host rocks. Crystallization ages obtained by U–Pb SHRIMP analysis of zircons from both the migmatites and the leucogranite veins indicate that the granulite facies metamorphism and the injection of veins were contemporary and occurred during slow cooling (6 °C My− 1) from the metamorphic peak at 2666 Ma to the granite solidus at 2636 Ma.The large discrepancy (about 50%) between the volume of anatectic melt produced locally (in situ) and the observed volume of leucogranite in the terrain indicates that anatectic melt accumulated in the Opinaca Subprovince; it is an injection complex. This finding that granulite facies terranes can be enriched in melt contrasts with the general view that granulite terranes are melt-depleted.The presence of an injection complex in the deep crust has several broad implications for the continental crust. 1) H2O released as the injected melt crystallized rehydrated the adjacent granulite facies rocks which is evident from orthopyroxene replaced by amphibole and biotite. Consequently, these deep rocks would be more fertile in a subsequent anatectic event than non-rehydrated residuum. 2) The retention of voluminous leucogranite in the injection complex has thermal consequences for the crust. First, the bulk composition of the crust containing the injection complex is necessarily more felsic, thus the budget of heat-producing elements (Th, U and K) is locally enhanced and radiogenic heat production is higher. Secondly, latent heat of crystallization is released at a deeper level where the leucogranite solidifies. The combined effect of both maintains the lower crust several tens of degrees warmer for longer period of time.

► A large volume of anatectic melt has accumulated in the deep middle crust.
► The terrane contains over 60% leucogranite, but ~ 10% is from in situ partial melting.
► Pervasive migration of leucogranite formed an injection complex of innumerable small dykes.