The geochemistry and petrogenesis of the Agnew Intrusion, Canada: a product of S-undersaturated, high-Al and low-Ti tholeiitic magmas

The 2100 m thick Agnew Intrusion (50 km(2)) in central Ontario, Canada, is a deformed, Palaeoproterozoic, layered leucogabbronoritic to gabbronoritic pluton that is believed to have intruded as a subvolcanic still between Arc haean granitic basement of the Superior Province and overlaying Palaeoprote rozoic flood basalts. Its emplacement was part of a major magmatic event in the region, which included the extensive Hearst-Matachewan dyke swarm, and was followed by rifting and accumulation of the thick Huronian Supergroup succession in the Southern Province. Litho- and chemostratigraphic analyses of the Agnew Intrusion show that it is the product of at least three major magma pulses, giving rise sequentially to a Marginal, Lower, and Upper Ser ies. The final and largest magma pulse produced a closed-system differentia ted sequence grading from olivine gabbronorites at the base to ferrosyenite s and alkali-feldspar granites at the top. Parental magmas of the Agnew Int rusion were S-undersaturated, high-Al and low-Ti tholeiites, exhibiting som e minor and chalcophile element compositions that are very similar to the m odel parent liquids proposed for the mafic portions of the Stillwater and B ushveld Complexes. Other mafic dyke groups that are spatially and temporall y associated with the Agnew Intrusion have strong petrological and geochemi cal similarities with the Hearst-Matachewan dyke swarm, but are not comagma tic with the intrusion. Possible mantle sources to the Agnew Intrusion incl ude the mantle residue after partial melting to form the Archaean greenston e sequences, and plagioclase-bearing mafic or ultramafic intrusions that ha ve ponded at the crust-mantle boundary during the Archaean. Partial melting in these mantle sources may have been induced by 'thermal' plumes.