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

Abstract

The 2100 m thick Agnew Intrusion (50 km2) in central Ontario, Canada, is a deformed, Palaeoproterozoic, layered leucogabbronoritic to gabbronoritic pluton that is believed to have intruded as a subvolcanic sill between Archaean granitic basement of the Superior Province and overlying Palaeoproterozoic 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 Series. The final and largest magma pulse produced a closed-system differentiated sequence grading from olivine gabbronorites at the base to ferrosyenites and alkali-feldspar granites at the top. Parental magmas of the Agnew Intrusion were S-undersaturated, high-Al and low-Ti tholeiites, exhibiting some minor and chalcophile element affinities with boninites. These magmas have major element compositions that are very similar to the model parent liquids proposed for the mafic portions of the Stillwater and Bushveld Complexes. Other mafic dyke groups that are spatially and temporally associated with the Agnew Intrusion have strong petrological and geochemical similarities with the Hearst-Matachewan dyke swarm, but are not comagmatic with the intrusion. Possible mantle sources to the Agnew Intrusion include the mantle residue after partial melting to form the Archaean greenstone sequences, and plagioclase-bearing mafic or ultramafic intrusions that have ponded at the crust-mantle boundary during the Archaean. Partial melting in these mantle sources may have been induced by 'thermal' plumes.