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Deformation, metamorphism, and magmatism in the East Athabasca mylonite triangle, northern Saskatchewan: Implications for the Archean and Early Proterozoic crustal structure of the Canadian Shield

Christopher Frederick Kopf, University of Massachusetts Amherst


The East Athabasca mylonite triangle (EAMT) is a well exposed region of granulite facies mylonitic rocks along the trace of the Snowbird tectonic zone in northern Saskatchewan. The region was deformed and metamorphosed in a lower crustal shear zone during an episode of intense magmatic intrusion and advective heating at 2.6 Ga. The EAMT can be divided into three major domains which are remarkably different in rock type, structural fabric orientation, and metamorphic history. Rocks in the southeastern domain consist mainly of early tonalitic gneisses and syntectonic mafic dikes that were metamorphosed at approximately 1.0 GPa and 700–800°C. The northwestern domain contains numerous syntectonic felsic and mafic intrusions which record metamorphic conditions of approximately 0.9 GPa, 700–800°C during cooling from higher igneous temperatures. Metamorphism in both of these domains appears to have occurred during localized heating and deformation associated with voluminous additions of new magma to an isostatically stable region of the lower crust. The third domain is dominated by highly deformed felsic gneisses in the north and overlying mafic intrusions in the south. The mafic rocks intruded during 2.6 Ga thermotectonism, possibly as part of a larger mafic underplate of the lower crust. They preserve spectacular coronitic growths of clinopyroxene, orthopyroxene, plagioclase and garnet that formed during and subsequent to 2.6 Ga deformation. Consistent reaction textures in the mafic rocks suggest that the corona assemblages developed during early deformation at 1.1–1.2 GPa and 750°C, followed by rapid heating to greater than 900°C (and possible decompression) during juxtaposition with the northeastern and southwestern domains. Late symplectite assemblages record retrograde conditions of 0.7–0.8 GPa and 700°C during rapid decompression of the zone during a previously unknown Early Proterozoic uplift event. This metamorphic sequence sets the mafic rocks apart from the felsic gneisses which record earlier high P-T metamorphism above 1000°C and 1.6 GPa. The metamorphic history of the mafic rocks reflects a significant thermal and magmatic event within the lower crust at 2.6 Ga, similar to that recorded in the southeastern and northwestern domains. All three domains show almost no affect of Early Proterozoic metamorphism or deformation. The intense episode of 2.6 Ga metamorphism and magmatism recorded in the East Athabasca mylonite triangle is not preserved in the adjacent regions of the Hearne and Rae Archean provinces. Extensive Archean granite-greenstone belts in the Hearne province suggest the presence of a sharp metamorphic boundary east of the EAMT. Rocks of the western Rae province are marked by domains of felsic intrusions and high metamorphic grade gneisses separated by steeply dipping shear zones. One domain in the Neil Bay region of Lake Athabasca records an intense period of deformation, magmatism, and high temperature metamorphism similar to that preserved in the EAMT, but at 1.9 rather than 2.6 Ga. The domainal partitioning of metamorphic and magmatic events among shear-zone-bounded regions may indicate the existence of a block-style architecture in the lower crust of the Canadian Shield similar to that proposed for the middle crustal terranes of the American southwest.

Subject Area


Recommended Citation

Kopf, Christopher Frederick, "Deformation, metamorphism, and magmatism in the East Athabasca mylonite triangle, northern Saskatchewan: Implications for the Archean and Early Proterozoic crustal structure of the Canadian Shield" (1999). Doctoral Dissertations Available from Proquest. AAI9932323.