A structual analysis of the Grenville Front Zone, Northeast Terrane, Labrador

Brown, Dennis Lewis (1990) A structual analysis of the Grenville Front Zone, Northeast Terrane, Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

The study area, located in western Labrador near the projected intersection of the front zones of the Grenville and Hudsonian orogens, occupies part of Gagnon terrane in the Parautochthonous Belt of the Grenville Province. Rocks in the area consist principally of supracrustal units of the Early Proterozoic Knob Lake Group, and a newly recognized, tectonically overlying unit, the Equus Lake formation, both of which are intruded by Middle Proterozoic gabbros of the Shabogamo Intrusive Suite. -- The structural elements in the rock units record evidence of a polyorogenic history that is attributed to NW-trending structures of the ca. 1800 Ma Hudsonian orogeny that were overprinted by NE-trending structures of the ca. 1000 Ma Grenvillian orogeny. -- Grenvillian deformation and metamorphism resulted in the formation of a relatively deep level, north-verging foreland fold and thrust belt. Four structural domains have been defined based on variations in morphology and orientation of structural elements, and internal domain geometry. Each domain is interpreted to be a structurally distinct thrust bound unit or group of units that has undergone a unique deformational history. The map pattern indicates a complex stacking order of the domains, and of thrust slices within domains, that involved development of northwest- to north-directed fold nappes followed by out-of-sequence thrusting. -- Metamorphic grade varies from lower to upper greenschist facies. Plagioclase-amphibole geothermometry shows qualitatively that there is an increase in Grenvillian metamorphic grade from northwest to southeast across the area. Quantitative garnet-biotite geothermometry indicates that the Grenvillian metamorphic grade attained temperatures of 450 ﾟC in the southern part of the area. Inversion of temperature with structural height in the thrust stack is a well known feature from higher level thrust stacks and implies thrusting post-dated the metamorphic peak. -- Documentation of the northern margin of Gagnon terrane as a NW-directed metamorphic fold and thrust belt corroborates similiar interpretations from elsewhere along the Grenville Front, and is in accord with the models of the Grenville Province as a collisional orogen.

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