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 maficto ultramafic intrusions crosscutting the Povungnituk Groupof the Cape Smith Fold Belt in New Quebec. The intrusions aremainly in the form of blade-shaped dikes that penetrate a sediment-richhorizon in the middle of the Beauparlant Formation and terminatedownward against massive basalts of the lower Beauparlant Formation.Significant accumulations of magmatic sulfide occur at the basalterminations of the dikes. At stratigraphic levels above theBeauparlant Formation the intrusions appear as broad dikes orsills within the Nuvilik Formation, below the mineralized lavaflows and subvolcanic intrusions of the Raglan Formation. TheExpo Intrusive Suite and the mineralized bodies of the RaglanFormation are probably coeval and comagmatic with the overlyingChukotat Group. Post-emplacement folding has exposed the ExpoIntrusive Suite over about 5 km of structural relief, revealingthe basal sulfide concentrations where dike segments terminateon the flanks of anticlines. The parent magma as preserved inchilled margins and narrow dikes was a picrite containing 17wt % MgO (i.e. komatiitic basalt) and slightly depleted in Th,U and Nb relative to middle and heavy rare earth elements. Thecompositions of ultramafic cumulate rocks within the intrusionsare strongly enriched in Th, U and Nb relative to heavy rareearth elements, reflecting assimilation of the enclosing basaltsand metasediments. Modeling of the assimilation process suggeststhat the picritic magma was capable of assimilating masses ofbasalt or sediment up to 50% of the original mass of magma.Assimilation of 10% of a mixture of basalt and sediment causedthe magma to become sulfide-saturated, and was accompanied bythe crystallization of masses of ultramafic cumulates approximatelyequal to the mass of rock assimilated. The presence of dikeswhose chilled margins resemble uncontaminated primary magmasbut that contain abundant cumulates recording wholesale assimilationof host-rocks indicates that the process of assimilation andfractional crystallization required to produce continental tholeiitesfrom picritic parent magmas may not require the presence oflong-lived magma chambers, but can occur during transport alongdikes and reaction with wall-rocks. KEY WORDS: komatiite; Expo Intrusive Suite; assimilation; fractional crystallization; sulfide mineralization