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

Along the northeastern margin of the Grenville Province in Canada, crust of Labradorian age (1.71-1.62 Ga) was variably reworked during northward (pre sent day coordinates) propagation of the 1.20-1.00 Ga Grenvillian Orogen. T o provide constraints on the nature of Grenvillian reworking, we describe p reviously unreported field relationships and present ten new U-Pb ages from a transect across Lake Melville terrane (LMT), which forms the leading edg e of the Allochthonous Polycyclic Belt in the study area. Exhumation of gra nulites on the northern margin of LMT, along the Rigolet thrust zone, occur red during the interval 1617-1610 Ma and was therefore a late Labradorian f eature. Approximately 120 m.y. after the Labradorian Orogeny, during the in terval 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, t ogether with a revised U-Pb baddeleyite and zircon age of 1472 +27/-21 Ma f or the Michael gabbro, suggests that Pinwarian ensialic are magmatism was c oeval with the emplacement of the Michael and Shabogamo gabbro dyke swarms. The data also indicate that Pinwarian magmatism was not limited to the Pin ware terrane, but occurred throughout the entire Grenville Province in east ern Labrador, implying that the region was already accreted to the Laurenti an margin prior to the Grenvillian Orogeny. The principal phase of Grenvill ian deformation and uppermost-amphibolite facies metamorphism in LMT occurr ed between 1.08 and 1.05 Ga and appears to have been focused between the Ri golet shear zone and the anorthosite-dominated Mealy Mountains terrane. Gre nvillian amphibolite facies reactivation of the Rigolet shear zone was limi ted 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 fol ding and belt-parallel transposition, minor thrusting, followed by localize d strike-slip shearing at the northeastern and southwestern margins. (C) 20 00 Elsevier Science B.V. All rights reserved.