Structural, metamorphic and tectonic studies in Central Gagnon Terrane, Grenville Province

Schwarz, Steven H. (1998) Structural, metamorphic and tectonic studies in Central Gagnon Terrane, Grenville Province. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Gagnon terrane, comprising the lower deck of the Parauthochthonous belt in northeastern Grenville Province, is a metamorphic fold-thrust and nappe belt consisting of Paleoproterozoic supracrustal rocks (Knob Lake Group) and their reworked Archean basement (Ashuanipi Metamorphic Complex) that is tectonically overlain by the Molson Lake terrane. This thesis comprises a structural-metamorphic-geochronologic study of an area in the central Gagnon terrane in eastern Québec, some 80km south of the Grenville Front. -- The structural component of the study involved the division of the area into thrust sheets utilizing stratigraphic, younging and fabric criteria, evaluation of the structural architecture, and interpretation of the structural evolution. Ductile thrusts bounding the thrust sheets have steep dips in the northern part of the map area, whereas those farther south have intermediate dips towards the north and a southerly vergence. This southerly vergence is in contrast to the well-developed northerly (-northwesterly) vergence that has been reported previously in the northern Gagnon terrane, and is interpreted in terms of backthrusting due to the incorporation of a major wedge of basement rocks into the thrust stack during its construction. -- Overall within the study area, there is evidence for four phases of deformation (D₁-D₄). D₁ involved development of the regional, penetrative foliation (S₁), that is subparallel to relict compositional layering (S₀) in most outcrops. F₁ folds vary from outcrop- to map-scale; large-scale F₁ folds had approximately E-W trending axes and are overturned towards the south or recumbent in different thrust sheets. In the northern thrust sheets, D₁ fabrics are refolded by outcrop- and map-scale F₂ folds with variably penetrative S₂ fabrics and generally SE-plunging axes. F₃ folds, previously unrecognized within Gagnon terrane, are local small-scale recumbent structures that occur in the northern thrust sheets and are interpreted to have developed as a result of gravitational collapse of the overthickened, thermally-weakened crust. F₄ folds are large-scale upright variably plunging cross-folds best developed in the southern part of the study area. Overprinting of folds of different generations has led to a wide range of structural interference patterns on all scales within the map area. -- Metamorphic rocks within the study area are composed of amphibolite facies mineral assemblages, and evidence of partial melting in rocks of pelitic and quartzofeldspathic composition is widespread. Field and microstructural relationships indicate that peak metamorphism and porphyroblast growth occurred syn- to post-D₁ and pre- to syn D₂, although there are local variations according to thrust sheet. The Lac Don thrust sheet in the northern part of the map area contains the assemblage Qtz-Ms-Grt-Bt-Pl±Kfs-L, whereas the Lac Gull thrust sheet farther south contains the assemblages Qtz-Kfs-Ky-Bt-Grt-L or Qtz-Ms-Kfs-Ky-Bt-Grt-L. Garnet zoning profiles from the Lac Don thrust sheet exhibit relict growth zoning that was significantly altered by post-peak re-equilibration, suggesting that temperature remained high for a prolonged period of time and that exhumation was relatively slow in this area. In contrast, garnets in the Lac Gull thrust sheet exhibit well-preserved growth profiles with very narrow retrograde rims, suggesting a hairpin shaped P-T path as a result of rapid, tectonically-driven exhumation. P-T estimates from both samples are in the range of 710-800°C and between 12-15 kbars. -- Thrusts faults are locally intruded by pegmatites, one of which shows evidence of syntectonic (post-D₁/pre-D₂) intrusion. This pegmatite has been dated by the U/Pb zircon method at 995 ± 2 Ma, thus providing the first direct estimation of the age of peak metamorphism and crustal thickening within this part of Gagnon terrane. -- These structural, metamorphic and geochronologic data are compatible with existing tectonic models for the northeastern part of Gagnon terrane in which NW-directed transport of a thrust wedge (Molson Lake terrane) over its foreland (Gagnon terrane) resulted in the progressive accretion of Gagnon terrane to the base of a thickening thrust wedge. However, the data from this thesis suggest that the tectonic evolution in central Gagnon terrane differed significantly in detail from that farther north. Metamorphic grade (both P and T) is higher, map-scale recumbent fold nappes are present, and northerly vergence and back-thrusting, due to the incorporation of a thick slice of basement rocks into the thrust pile, is important. These features have been incorporated into a revised tectonic model for the evolution of Gagnon terrane.

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