The evolution of the Grenville Province in eastern Labrador, Canada

Abstract

Rifting at c. 1.71 Ga separated the Mealy Mountains terrane from pre-Labradorian Laurentia and resulted in a short-lived (backarc?) basin, followed by southward subduction giving rise to 1.68-1.66 Ga Labradorian calc-alkaline arcs, partly built on older crust in the Mealy Mountains terrane. The arcs and subduction were also short lived, terminating when accretion against the Makkovikian and Eastern Churchill provinces occurred at 1.65 Ga. The Trans-Labrador batholith developed as a result of crustal thickening during suturing of the 1.68-1.66 Ga arc-related rocks to pre-Labradorian Laurentia. Further bimodal magmatism, in response to crustal thickening, continued until c. 1.62 Ga, followed by waning granitic magmatism until 1.60 Ga. The Labrador Orogen and older rocks in the Mealy Mountains terrane are flanked to the south by Pinwarian (1.51-1.45 Ga) granitoid rocks, possibly the inboard manifestation of a northward-subducting continental-margin arc at the southern margin of Laurentia. Extension, mostly coincident with the 1.71 Ga zone of weakness, started at c. 1.45 Ga. Although rifting was successful further west, it is only represented by basaltic magmatism (Michael and Shabogamo gabbros) along a linear zone in Labrador. Similar tectonism characterized the same zone periodically for at least the next 150 million years during which time the 1.27 Ga Harp-Nutak dykes were emplaced and slightly younger rocks of the Seal Lake Group formed north of the Grenville Province. After c. 1.23 Ga tectonic conditions changed, partly in response to Elzevirian accretion of arc terranes preserved in the southwest Grenville Province, that are speculated to have once also existed south of the exposed eastern Grenville Province. Alternating compressional and extensional conditions terminated at c. 1.0 Ga during collisional orogenesis associated with the final stages of development of the Grenville Orogen. Grenvillian deformation in Labrador (1.08-0.97 Ga) resulted in northwesterly thrusting, the effects of which are focused in the Exterior Thrust Belt, and widespread plutonism in the Interior Magmatic Belt. Thrusting in the Exterior Thrust Belt is sited along the zone of 1.71 and 1.43 Ga rifting. Uplift and cooling continued until 0.90 Ga as crustal stability was gradually achieved.