Mapping crustal terrane boundaries in Manicouagan Quebec, of the Grenville Province: Characterization of isotopic signatures from differing crustal sources and evidence from aeromagnetic data

Abstract

<p>The Grenville Province is alGa orogenic belt, composed of much older terranes whose age and extent require mapping in order to gain a better understanding of its geological evolution. Manicouagan, Quebec, in the Grenville Province, has experienced several orogenic events, as well as being the site of a 214 Ma hypervelocity impact event, which has further complicated its geological structure. By analyzing surface samples from this area, as well as driB core of the country rocks involved in the impact, the original age of crustal formation for the various terranes can be calculated, thus revealing the evolutionary history of the Manicouagan area.</p> <p>Nd isotopes are resistant to metamorphic disturbances, therefore permitting accurate calculations of original crustal formation ages. Nd-isotopic analysis of granitoid orthogneisses, has identified three major crustal formation age groups in the area. These are: Mesoproterozoic (1.58 Ga), Paleoproterozoic (1.86 Ga), and Archean (2.8 Ga). These average depleted mantle model ages (TOM) correspond well with isochron reference lines, supporting their validity as actual geologic events.</p> <p>The spatial distribution of age data results indicate that the Archean basement extends farther south on the western side of the Manicouagan reservoir than previously thought, whereas the eastern side is dominated by Paleoproterozoic crust. In contrast, the Manicouagan Imbricate Zone (MIZ), located between the Archean and Paleoproterozoic crustal terranes, as a wider scatter of Nd data attributed to mixing between Archean and Proterozoic sources. This crustal terrane has been limited to within the interior of the reservoir in the south, and extends just north of the reservoir. The location of the boundary between the Archean and Paleoproterozoic terranes, the Allochthon Boundary Thrust (ABT), was largely agreed upon to the east and west of the impact, but its position was highly ambiguous near the impact itself. New Nd analysis shows that this boundary crosses onto the island on the NW side of the reservoir, and exits to the west of the previously established Cryptic Shear Zone (CSZ) at the southern end of the impact. The boundaries of these distinctive crustal terranes are further supported through aeromagnetic evidence.</p> <p>The 214 Ma hypervelocity event resulted in the instantaneous melting of target rocks located beneath the impact. Isotopic and elemental geochemistry was employed to characterize the composition of the meltsheet. These data show that the melt sheet was largely derived from the MIZ, but cannot rule out a small component of underlying Archean crust. Further isotopic, trace element and major elemental analyses are needed in order to fully decipher the target rock composition.</p>