Isotopic and geochemical investigations of Precambrian continental crust in the Torngat Orogen, northeastern Canada: Constraints on the mechanisms of Precambrian crust formation and on the Early Proterozoic assembly of northeastern Laurentia.

摘要

The Torngat Orogen (TO) in Northern Labrador, Canada, formed during Early Proterozoic continental collision between the Archean Nain and Rae provinces. The TO preserves Early Proterozoic and Archean rocks which offer an excellent opportunity to study crust-mantle evolution and collisional tectonics during the Early Precambrian. This dissertation presents the results of isotopic and geochemical investigations on Archean and Early Proterozoic metaigneous rocks from this region. The studies were designed to: (a) document the petrogenesis of these rocks; (b) determine the lateral extent of Early Proterozoic metaigneous rocks and underlying Archean basement gneisses; and (c) evaluate spatial variations in the isotopic compositions of Early Proterozoic metaigneous rocks with respect to current models for subduction zone geometry.; The study has shown that (a) the Burwell domain (BD) is underlain by Archean gneisses which differ laterally in isotopic and geochemical composition and are "Nain-like" in the northeast, and "Rae-like" in the southwest. This suggests that the BD is not allochtonous with respect to adjacent Archean Provinces and contains elements of both; (b) Early Proterozoic metaigneous rocks occurring within the BD and Nain Province are geochemically similar to Phanerozoic arc magmas, however spatial variations in their {dollar}rmepsilon Ndsb{lcub}(i){rcub}{dollar} values do not increase inland (eastward), as would be expected for the proposed eastward dipping subduction models. It is suggested here that the spatial distribution of {dollar}rmepsilon Ndsb{lcub}(i){rcub}{dollar} reflect emplacement of the arc magmas into Archean crust which was underplated by a variably thick column of basalt, possibly emplaced during a period of incipient rifting prior to subduction. Although there is currently no direct evidence for this, there are several pieces of indirect evidence that rifting may have preceeded arc magmatism in the eastern BD.; In previous models for the assembly of NE Laurentia, the BD represented an independent crustal terrane which was accreted to the Nain Province during Proterozoic collision. The results from this study suggest that this "foreign" terrane did not exist, and hence that revisions to the models are necessary. This research further confirms that modem-style plate tectonic processes controlled Early Proterozoic crustal amalgamation in this segment of Northeastern Laurentia.