VERTICALLY-LAYERED MAFIC INTRUSIONS IN THE EASTERN AND WESTERN GRENVILLE PROVINCE: ORIGIN AND IMPLICATIONS

Once subjected to orogenesis, collapsed mafic-silicic layered intrusions (MASLI) and mafic intrusions with primary vertical layering are respectively difficult to distinguish from syn-tectonic mafic-felsic sheet intrusions emplaced in steep deformation zones and from folded or tilted horizontally-layered intrusions. Their distinction is critical to the understanding of mafic magmatism in the Grenville Province and to the development of sound geological models to support mineral exploration. The 1.37 Ga Matamec intrusive complex (eastern Grenville) comprises a series of massive gabbronorite and foliated porphyritic monzonite sheets, steeply folded into a large synform. The sheets, their magmatic and solid-state foliation and the intraplutonic shear zones are steeply dipping and parallel to each other. They are cut by non-deformed 1.35 Ga gabbro dykes, hosts to massive sulphide Ni-Cu-Co mineralization. Dyking relationships record syn-intrusion deformation focussed in felsic sheets while load casts and flame structures in non-deformed sheets with comingling textures require the sheets to have been originally horizontal. The synformal structure is attributed in part to the collapse of a MASLI-type intrusion. Late-stage mafic dykes brought up magmatic sulphides along the flanks of the body and across its fold hinge. In contrast, the concordant, elongate and sub-vertical shape of the 1.165 Ga Chevreuil monzonite-to-gabbro sheet intrusions (western Grenville) results from syn-tectonic magma emplacement along steeply-dipping, crustal-scale deformation zones. Coeval cylindrical gabbro stocks straddle the opposite flanks and the hinge of a regional antiform. In spite of these distinct settings, they all share systematically subvertical modal layering, igneous foliation and trough bedding. The stocks record magma flow and side-wall crystallization in a vertical cylindrical conduit, not bottom-up accumulation with later folding and tilting of horizontal beds. These stocks occur at the intersection of conjugate shears; if active at the time of intrusion, the shear system would provide potentially local extensional settings for magmatic sulphide ponding at the base of the stocks.