U–Pb constraints for the plutonic and tectonometamorphic evolution of Lake Melville terrane, Labrador and implications for basement reworking in the northeastern Grenville Province

Abstract

Along the northeastern margin of the Grenville Province in Canada, crust of Labradorian age (1.71–1.62 Ga) was variably reworked during northward (present day coordinates) propagation of the 1.20–1.00 Ga Grenvillian Orogen. To provide constraints on the nature of Grenvillian reworking, we describe previously unreported field relationships and present ten new U–Pb ages from a transect across Lake Melville terrane (LMT), which forms the leading edge of the Allochthonous Polycyclic Belt in the study area. Exhumation of granulites on the northern margin of LMT, along the Rigolet thrust zone, occurred during the interval 1617–1610 Ma and was therefore a late Labradorian feature. Approximately 120 m.y. after the Labradorian Orogeny, during the interval 1489–1474 Ma, dioritic to granitic plutons inferred to be related to Pinwarian (1.51–1.47 Ga) magmatism, were emplaced in northern LMT. This, together with a revised U–Pb baddeleyite and zircon age of 1472+27/–21 Ma for the Michael gabbro, suggests that Pinwarian ensialic arc magmatism was coeval with the emplacement of the Michael and Shabogamo gabbro dyke swarms. The data also indicate that Pinwarian magmatism was not limited to the Pinware terrane, but occurred throughout the entire Grenville Province in eastern Labrador, implying that the region was already accreted to the Laurentian margin prior to the Grenvillian Orogeny. The principal phase of Grenvillian deformation and uppermost-amphibolite facies metamorphism in LMT occurred between 1.08 and 1.05 Ga and appears to have been focused between the Rigolet shear zone and the anorthosite-dominated Mealy Mountains terrane. Grenvillian amphibolite facies reactivation of the Rigolet shear zone was limited to its upper structural levels only, with the central and lower levels preserving Labradorian fabrics and granulite facies assemblages. High-grade Grenvillian deformation within LMT was dominated by tight to isoclinal folding and belt-parallel transposition, minor thrusting, followed by localized strike–slip shearing at the northeastern and southwestern margins.