Date of Award
Doctor of Philosophy (PhD)
Paul M. Clifford
A detailed study of the tectonic and petrogenetic history of Archean rocks at Kenora has unravelled a complex deformational/intrusive history. This is integrated with previous work into a tectonic evolutionary model for the English River Subprovince.
The earliest Kenora Area rocks were mafic volcanics, cogenetic sills and minor ultramafic, intermediate and sedimentary rocks. The mafic rocks are tholeiitic with island arc/ocean floor characteristics. Compositional data are compatible with shallow level fractionation from a peridotite source.
Biotite tonalite gneisses and the more homogeneous Melick Tonalite, for which a sedimentary protolith is rejected are interpreted as plutonic rocks emplaced into the supracrustal sequence. Rb-Sr ages are,
REE patterns in the tonalitic rocks are modelled by 10% melting of a two-pyroxene garnet granulite of tholeiitic composition.
Granitic pegmatoid gneisses are interpreted as deformed pegmatites injected into tonalitic gneiss and amphibolite. Bulk compositional data are compatible with fractionation from a tonalitic magma, but unusual LREE-depleted REE patterns accessory phases precipitate before residual liquid crystallizes.
A suite of calc-alkaline mafic/mafelsic dikes were injected, over an extended period, into the gneisses and some granitic plutons. Overlapping time of emplacement and calcalkaline affinities indicates a link with the granitic plutons.
The Dalles Granodiorite, the only granitic pluton studied in detail, consists of intergradational pink and grey phases which together define an Rb-Sr age of 2630 ± 10 Ma, initial ratio 0.7014 ± 1. REE patterns in the grey phase can be modelled by 10% melting of hornblende-bearing garnet granulite or, ignoring a model positive Eu anomaly, quartz eclogite. The pink phase is modelled by 70% fractional crystallization of the grey phase under conditions of high fO₂.
A severe compression ( ), now recorded as intrafolial, rootless isoclinal folds, post-dates the tonalitic rocks. Subsequent injection and further deformation (D₂) produced a well-banded gneissic rock. Major F₂ folds can be delineated from the geometry of minor F₂ structures. During deformation the area was subjected to upper amphibolite grade metamorphism (650-750ºC, 0.4-0.7 GPa) with K-feldspar megacrysts and garnet developing in the deeper levels. Major F₂ folds are refolded in a rim synform (D₃) which developed synchronously with the emplacement of the Dalles Granodiorite, an event which also accentuated tonalite gneissosity and created a plunge culmination uplifting part of the deeper K-feldspar megacryst/garnet structural level. D₄ deformation produced open, upright folds and caused buckling of dioritic dikes and pegmatites. D₅ and D₆ deformations invoIve minor faulting and joint development.
The tectonic model formulated for the English River Subprovince is divisible into five parts:
(i) Volcanic-plutonic cycle I (not present at Kenora). Tonalitic gneisses from Lac Seul (ca. 3050 Ma) and undated gneisses at Cedar Lake were formed. Amphibolite enclaves may be the only surviving relics of a pre-existing supracrustal sequence.
(ii) Volcanic-plutonic cycle II. The Kenora Area supracrustal remnants and tonalitic gneisses (2760 Ma) and the undated Clay Lake Granitoid Suite, Twilight (metasedimentary) Gneisses and supracrustal rocks at Cedar Lake and a trondhjemite-granodiorite unit at Lac Seul (2780 Ma) were generated during this cycle, as were the Ear Falls - Manigotagan Gneiss Belt metasediments.
(iii) Major tectonothermal event (2680 Ma). Regional metamorphism resulted in upper amphibolite to granulite mineral assemblages. An intense layer normal compression event at Cedar Lake caused tectonic interleaving of the Cedar Lake Gneiss 'basement' and the Twilight Gneiss 'cover' sequence.
(iv) Diapiric granite event (2650 Ma). The Dalles Granodiorite (2630 Mal at Kenora and the undated Cliff Lake Granodiorite at Cedar Lake are local representatives of this event. Sialic material at the present erosion level was doubled and the English River Subprovince was elevated relative to the adjoining Wabigoon Subprovince.
(v) Late events (2550 Ma). Sporadic injection of minor intrusions and weak deformation conclude the tectonic evolution of the English River Subprovince.
Gower, Charles Frederick, "The tectonic and petrogenetic history of Archean rocks from the Kenora Area, English River Subprovince, northwest Ontario" (1978). Open Access Dissertations and Theses. Paper 3867.