Petrogenesis of Archean granitoid plutons from the Kenora area, English River Subprovince, Northwest Ontario, Canada

Abstract

In the Kenora area, the emplacement of granitoid plutons and associated minor intrusions into older tonalitic gneiss and remnant mafic supracrustal rocks, represents the second half of a protracted history during the Kenoran Orogeny. The oldest granitoid pluton is the Marginal granodiorite (2735 ± 65 Ma) which separates gneiss of the English River Subprovince from supracrustal rocks of the Wabigoon Subprovince. This unit has a narrow, elongate form and a gradational boundary with the gneiss. These features, and the unit's age, initial Sr ratio and REE patterns that are analytically indistinguishable from those of the tonalitic gneiss, are reasons for interpreting the Marginal granodiorite as being derived by ‘frictional reconstitution’ of tonalitic gneiss, resulting from heating during movement along the Kenora fault. The Melick tonalite (2630 ± 50 Ma), and the Lulu and Dalles granodiorites (2630 ± 10 Ma) are interpreted as the next additions to the area. A correlation between decreasing age, increasing initial Sr ratio, increasing size of intrusive body, and progressive depletion of heavy REE may indicate that the Kenora tonalitic gneiss, Melick tonalite, Lulu granodiorite and Dalles granodiorite represent a continuum of genetically related plutonism. The changes in heavy REE suggest that progressively younger source rocks (all comparable to Archean tholeiite) underwent transformation from high-pressure granulite to eclogite. The Dalles granite is modelled by fractional crystallization of the Dalles granodiorite. The youngest plutons are the Muriel diorite-granodiorite (2660 ± 25 Ma) and the Austin granite (2631 ± 63 Ma), both interpreted as being derived from upper crustal rocks. A sequence of dioritic rocks occupies an intermediate structural and lithological position between the amphibolite-tonalite gneiss and the Muriel granodiorite. The model advanced is that the diorites represent the ‘roots’ of major diorite plutons, and indicate derivation by partial melting of amphibolite (and perhaps also low pressure granulite) and tonalitic gneiss. The melt, and some residual material, was mobilized upward along structurally controlled paths. The Austin granite is a potassic pluton with a higher initial Sr ratio than other units in the area. It is interpreted as being derived by extensive partial melting of rocks comparable to the Lulu or Dalles granodiorite.