Crustal contamination of picritic magmas during transport through dikes: The expo intrusive suite, Cape Smith Fold Belt, New Quebec

The Proterozoic Expo Intrusive Suite comprises a series of mafic to ultramafic intrusions crosscutting the Povungnituk Group of the Cape Smith Fold Belt in New Quebec. The intrusions are mainly in the form of blade-shaped dikes that penetrate a sediment-rich horizon in the middle of the Beauparlant Formation and terminate downward against massive basalts of the lower Beauparlant Formation. Significant accumulations of magmatic sulfide occur at the basal terminations of the dikes. At stratigraphic levels above the Beauparlant Formation the intrusions appear as broad dikes or sills within the Nuvilik Formation, below the mineralized lava flows and subvolcanic intrusions of the Raglan Formation. The Expo Intrusive Suite and the mineralized bodies of the Raglan Formation are probably coeval and comagmatic with the overlying Chukotat Group. Post-emplacement folding has exposed the Expo Intrusive Suite over about 5 km of structural relief, revealing the basal sulfide concentrations where dike segments terminate on the flanks of anticlines. The parent magma as preserved in chilled margins and narrow dikes was a picrite containing ∼17 wt % MgO (i.e. komatiitic basalt) and slightly depleted in Th, U and Nb relative to middle and heavy rare earth elements. The compositions of ultramafic cumulate rocks within the intrusions are strongly enriched in Th, U and Nb relative to heavy rare earth elements, reflecting assimilation of the enclosing basalts and metasediments. Modeling of the assimilation process suggests that the picritic magma was capable of assimilating masses of basalt or sediment up to 50% of the original mass of magma. Assimilation of ∼10% of a mixture of basalt and sediment caused the magma to become sulfide-saturated, and was accompanied by the crystallization of masses of ultramafic cumulates approximately equal to the mass of rock assimilated. The presence of dikes whose chilled margins resemble uncontaminated primary magmas but that contain abundant cumulates recording wholesale assimilation of host-rocks indicates that the process of assimilation and fractional crystallization required to produce continental tholeiites from picritic parent magmas may not require the presence of long-lived magma chambers, but can occur during transport along dikes and reaction with wall-rocks