Metamorphic and structural evolution of Archean rocks in the Keskarrah Bay area, Point Lake, District of Mackenzie, N.W.T

Jackson, Valerie A. (1989) Metamorphic and structural evolution of Archean rocks in the Keskarrah Bay area, Point Lake, District of Mackenzie, N.W.T. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Keskarrah Bay area, Point Lake, N.W.T. is underlain by Archean rocks of the Slave Province that form two distinct lithotectonic elements: a sialic basement terrane, consisting of high-grade gneisses and granodiorite, and a supracrustal terrane, known as the Itchen Lake region supracrustal belt, comprising metavolcanic and metasedimentary rocks of the Yellowknife Supergroup. -- Conditions of regional metamorphism are documented mainly from metamorphic mineral assemblages in turbidites, which comprise greywacke-mudstone and subordinate iron formation and Fe-rich sediments. The sequential development of the key metamorphic index minerals chlorite, biotite, cordierite, andalusite and sillimanite in pelitic lithologies and garnet, staurolite or Ca and Fe-rich clino-amphibole in Fe-rich sediments occurred dominantly through continuous rather than discontinuous metamorphic reactions. The measured trends of increasing Mg:Fe ratios of the index minerals (cordierite > muscovite > chlorite > biotite > amphibole > staurolite > garnet) are used to demonstrate changes in AFM topology, from which the continuous reactions are interpreted. -- The pattern of metamorphic isograds is attributed to a single, progressive low pressure regional metamorphic event. Metamorphic grade increases both eastwards towards the Yamba batholith, and westwards towards the basement gneisses and the Pointless batholith. Prograde metamorphism in the supracrustal rocks was synchronous with retrograde metamorphism in the basement terrane. Peak metamorphic conditions, which reached a maximum of about 600°C +/- 50°C and 4.0 +/- 1.6 kbar, were attained toward the end of deformation. The P-T distribution throughout the area is consistent with the presence of a distant thermal dome that formed in the area of eventual batholith emplacement. -- Three or possibly four phases of deformation have been recognized in the supracrustal terrane. Proximity to basement is considered to have had an influence on the orientation and intensity of structures developed during the first two phases of deformation. East-west trending isoclinal folds (F₁) and a penetrative S₁ fabric produced during D₁ are recognized mainly along the western edge of the supracrustal belt. D₂ resulted in the formation of north-south trending main phase F₂ folds that refold F₁ and are overturned to the west, and an S₂ foliation, both of which predominate throughout much of the map area. Post-D₂ deformations produced crenulations of the S₂ foliation, an L-S fabric defined by elongate biotite porphyroblasts and southeast- and northeast-trending biotite schistosities (tentatively designated S₃ and S₄). These fabrics, which are well developed in central and eastern exposures of the metaturbidites, are unrelated to large scale folding. -- The polyphase deformation (Dx) in the basement terrane that led to the formation of the basement gneisses predated deposition of the Yellowknife Supergroup. After deposition of the Yellowknife Supergroup, the basement gneisses adjacent to the supracrustal rocks were deformed by north-trending mylonitic foliations and large scale folds, which are correlated with S₂ and F₂ respectively in the supracrustals. West-verging recumbent folds (pre-D₂?) in the gneisses may indicate the formation of nappes, which may have also affected the supracrustal rocks. -- The westward overturning of main phase F₂ folds in the supracrustal terrane is towards basement. A similar relationship is observed in parts of the southern Slave Province, thus the formation of those folds represents a major tectonic event in the slave Province. The suggested deformational history of the Itchen Lake region supracrustal rocks is that of a west-verqing fold-and-thrust belt.

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