Thermotectonic evolution of the Proterozoic-Archaean boundary in the Saglek area, northern Labrador

Mengel, Flemming Cai (1987) Thermotectonic evolution of the Proterozoic-Archaean boundary in the Saglek area, northern Labrador. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In the Saglek Fiord area, northern Labrador, sediments of the Lower Proterozoic Ramah Group occur as a north-trending fold belt which overlies the boundary between the Nain Province to the east and the Churchill Province to the west. The Nain Province is a polycyclic Archaean gneiss complex, consisting of tonalitic to granodioritic gueisses with remnants of supracrustal sequences. The metamorphic grade varies from amphibolite to granulite facies. In the Saglek area, rocks in the Churchill Province are mainly Archaean gneisses thoroughly reworked in a major north-south oriented transcurrent sinistral shear zone. Three tectonically juxtaposed north-trending lithotectonic terranes can be distinguished in the Churchill Province (from east to west): the Amphibolite Facies Terrane (AFT), the Granulite Facies Terrane (GFT), and the Tasiuyak Terrane (TT). -- The sediments of the Ramah Group were deposited unconformably on the peneplained Archaean basement, as evidenced by a well preserved regolith and basal conglomerate. Subsequently, the Hudsonian Orogeny caused deformation and metamorphism of the Ramah Group. -- Two thermotectonic events of Proterozoic age can be distinguished in the Saglek area. Using Proterozoic dykes (2300-2400 Ma) as time markers, the structural evolution can be subdivided into "Early Proterozoic" and "Hudsonian" orogenic events. The Early Proterozoic orogeny is characterized by transcurrent shearing and counterclockwise rotation of structures into parallelism with the north-south oriented sinistral shear zone. Proterozoic dykes clearly demonstrate the rotation; these are east-west oriented in the Nain Province, and rotate towards a north-south orientation in the AFT and GFT. The GFT, TT and most of the AFT are located within the shear zone, and are characterized by well developed subhorizontal mineral lineations and steeply west-dipping planar (mylonitic) foliations. Rocks In the GFT and TT are in granulite facies, and are interpreted to correspond to the deeper levels of a major shear zone, while the AFT, in amphibolite facies, represents a higher level. The Ramah Group does not possess any structural overprints attributable to the transcurrent shearing, and deposition is thus believed to post-date Early Proterozoic thermotectonism. -- During the subsequent Hudsonian Orogeny, which is characterized by an east-west oriented compressive tectonic regime, east-directed thrusting caused a reversal of the metamorphic sequence established in the Early Proterozoic shear zone. Rocks of the GFT and TT were thrust eastwards over the AFT, which in turn was thrust over the Ramah Group causing deformation and metamorphism of the sedimentary sequence. -- Petrographic and thermobarometric studies of Early Proterozoic mineral assemblages in the GFT and TT show that evidence of both peak metamorphic conditions and subsequent decompression reactions is preserved. Highest P and T (10 kbar and 800ﾟC) are recorded by cores of coexisting minerals in equigranular aggregates, whereas rims record varying degrees of post-peak re-equilibration. Substantial decompression accompanied by only minor cooling (10 to 5 kbar and 800 to 650ﾟC) is shown by symplectites developed at garnet rims and between garnets and clinopyroxenes. These symplectites developed during erosion and uplift immediately following the Early Proterozoic tectonothermal event. -- Hudsonian metamorphic effects are restricted to the rocks of the Ramah Group and to minor retrogression of assemblages in the AFT. Microstructural evidence suggests that the metamorphic parageneses observed in the Ramah Group formed during a single metamorphic event. Hence the variations along the present erosional level represent a "metamorphic field gradient". Metamorphic grade in the Ramah Group increases from east to west and from north to south; the highest grade is recorded south of Saglek Fiord and corresponds to ca. 650°C/6.5 kbar. -- Key words: Labrador; Saglek area; Archaean-Proterozoic boundary; Early Proterozoic transcurrent shear zone; Hudsonian Orogeny; granulite facies; amphibolite facies; geothermometry; geobarometry; decompression reactions; symplectites; Ramah Group.

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