Mesoproterozoic basement of the Lac St. Jean Anorthosite Suite and younger Grenvillian intrusions in the Saguenay region, Québec: Structural relationships and U-Pb geochronology
Published:January 01, 2004
Claude Hébert, Otto van Breemen, 2004. "Mesoproterozoic basement of the Lac St. Jean Anorthosite Suite and younger Grenvillian intrusions in the Saguenay region, Québec: Structural relationships and U-Pb geochronology", Proterozoic Tectonic Evolution of the Grenville Orogen in North America, Richard P. Tollo, James McLelland, Louise Corriveau, Mervin J. Bartholomew
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The Chicoutimi Gneiss Complex was previously considered to host the huge 1160–1140 Ma Lac St. Jean anorthosite suite (20,000 km2), the 1082 ± 3 Ma Chicoutimi Mangerite, and the 1067 ± 3 Ma La Baie Granite. New geological mapping and geochronological data from the Chicoutimi Gneiss Complex now demonstrate clear correlations among some gneissic units and the partially deformed Chicoutimi Man-gerite and La Baie Granite. The remaining gneissic units in the Chicoutimi Gneiss Complex have been grouped into six new lithodems. The three oldest were generated during two distinct events. The Saguenay Gneiss Complex, which is one of these lith-odems and includes the Cap de la Mer Amphibolite unit dated at 1506 ± 13 Ma, is related to a widespread Pinwarian event. It was followed, 100 Ma later, by the 1391 +7/−8 Ma Cap à l'Est Gneiss Complex and the 1383 ± 16 Ma Cyriac Rapakivi Granite. Two younger lithodems are the 1155–1135 Ma Kénogami Charnockite and the 1150 ± 3 Ma Baie à Cadie Mafic-Ultramafic Suite, which are genetically related to the Lac St. Jean Anorthosite Suite. Finally, the 1045 ± 5 Ma Simoncouche Gabbro is the youngest Grenvillian igneous unit dated so far in the Saguenay region. The Lac St. Jean Anorthosite Suite, the Chicoutimi Mangerite, the La Baie Granite, and their host rocks (the Saguenay Gneiss Complex, the Cap à l'Est Gneiss Complex, and the Cyriac Rapakivi Granite) are everywhere in tectonic contact with each other. These contacts are part of the St. Fulgence shear zone, which is over 400 km long and several kilometers wide. Since the Simoncouche Gabbro outcrops within the St. Fulgence shear zone and is weakly deformed, it can be concluded that 1045 ± 5 Ma marked the end of the final movement along the St. Fulgence shear zone in the Saguenay region.