The role of granitic intrusions in the evolution of the Abitibi belt, Canada

Abstract

The Abitibi greenstone belt, Canada, constitutes a segment of a broad volcanic arc that experienced three distinct periods of volcanic activity peaking at 2730–2725, 2718–2714 and 2705–2700 Ma. During all three volcanic episodes, which were generated by subduction-related processes, mantle plume-derived komatiitic volcanic rocks were also added to the composite Abitibi arc. Plume activity resulted in local regional extension, and restricted later volcanism within the belt. North–south shortening produced thrusts and folds, followed by SE-trending dextral strike-slip faults between 2700 and 2692 Ma, the principal belt-wide deformation. Subsequent E–W dextral strike-slip faulting, limited to the major faults, continued to 2690–2680 Ma (Destor–Porcupine fault) and even to 2680–2670 Ma (Cadillac–Larder fault). Extensional structures resulting from the thickened crust developed around 2660 Ma. Granitic intrusions are associated with all the volcanic episodes and phases of deformation, and have been used to constrain the age of many of these events. Their role in stabilizing, and adding to the volcanic crust is perhaps less well appreciated. Large synvolcanic tonalite-dominated intrusions of the tonalite–trondhjemite–granodiorite (TTG) suite are associated with the three major periods of volcanic activity of the Abitibi greenstone belt, and are genetically related to the volcanism. These synvolcanic intrusions occur both as high-level, flat, cauldron subsidence complexes and as batholithic masses at the base of the oceanic crust. Tonalite gneiss, dated from 2714 to 2703 Ma, occurring in structural culminations, is considered to be arc-underplated material. The gneiss crystallized at depth, and, in contrast to the other tonalites, does not appear to have intruded volcanic rocks. The gneisses also record the earliest compressional deformation. Numerous intrusions of island arc affinity, both the TTG and monzodiorite (MZD) suites form stocks and small plutons emplaced during the late phases of major deformation. These are more numerous than voluminous, and most of the tonalite end members of the suites probably intruded deeper within the sequence, retarded by the now-thickened crust. Large tonalite intrusions dominate the uplifted deeper sections adjacent to the Abitibi belt in the Kapuskasing uplift and Grenville Parautochthon, and presumably make up much of the deep crust of the Abitibi belt imaged in Lithoprobe seismic profiles. Small stocks and dyke complexes characterize the late, localized strike-slip deformational phase. These intrusions are porphyritic syenites and related rocks, including some alkaline compositions. Garnetiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2660 Ma. Thus, plutonic rocks are a volumetrically major overall component of the Abitibi belt and were implicated in every phase of its development, from early volcanic arc magmatic activity, through the various stages of deformation, to final stabilization as continental crust.