Geology and zircon geochronology of the Acasta Gneiss Complex, northwestern Canada: New constraints on its tectonothermal history

The Acasta Gneiss Complex of northwestern Canada contains the oldest known crustal rocks on Earth. Here we present a detailed geological map of the main area of the complex (around the sample locality of the oldest known rocks) and detailed sketch maps of critical geological outcrops. The geological map shows that the complex is divided, by a northeast-trending fault, into eastern and western domains. The eastern area is comprised from quartz dioritic-gabbroic gneisses and multi-phase tonalitic-granitic gneisses. The western area is comprised of layered quartz dioritic-dioritic and tonalitic-granitic gneisses and younger foliated granitic intrusions. The detail field observations reveal at least five tectonothermal events in the eastern area: (1 and 2) emplacement of mafic-intermediate magma (protolith of the quartz dioritic gneiss) and emplacement of felsic magma (protolith of the older felsic gneiss); (3) metamorphism to produce the gneissic structures of the felsic gneiss and quartz dioritic gneiss; (4) intrusion of felsic magma (protolith of the younger felsic gneiss), causing anatexis in some parts; (5) metamorphism and deformation to produce the gneissic structure of the younger felsic gneiss. In contrast, at least four tectonothermal events have been recognized in the western area: (1 and 2) emplacement of the protolith to the mafic-intermediate and felsic gneiss; (3) metamorphism and deformation to form the gneissic and layered structures; (4) intrusion of the granite sheet (the protolith of the foliated granite); (5) metamorphism and deformation of all lithologies. To constrain the timing of the tectonothermal events, we have carried out U-Pb dating combined with cathodoluminescence imagery on zircon extracted from the gneisses and foliated granites. Our data reveal at least four tonalite-granite emplacement events in the eastern area, at ca. 3.94, 3.74-3.73, 3.66 and 3.59 Ga, and tonalite emplacement at ca. 3.97 Ga and granite intrusion at 3.58 Ga in the western area. The field relationships between the felsic and quartz dioritic gneisses in the eastern area demonstrate that the two quartz dioritic gneiss protoliths were emplaced prior to 3.59 and 3.66 Ga, respectively. These results confirm findings of previous zircon geochronology that the protolith ages of Acasta gneisses are 4.03-3.94, 3.74-3.72 and ca. 3.6 Ga. In addition, our comprehensive field observations, zircon internal structures and zircon U-Pb dating clearly demonstrate that the early Archean Acasta rocks suffered anatexis/recrystallization, coincident with the emplacement of younger felsic intrusions, and that the 3.59 Ga granitic gneiss protolith in the eastern area contains zircon xenocrysts with ages up to 3.9 Ga. In this context, we also discuss the tectonothermal evolution of the Acasta Gneiss Complex on the basis of these results and those from previous studies, and its implications for radiogenic isotopic studies.