U-PB AGE OF THE SEAL-LAKE-GROUP, LABRADOR - RELATIONSHIP TO MESOPROTEROZOIC EXTENSION-RELATED MAGMATISM OF LAURASIA

The Seal Lake Group of eastern Labrador unconformably overlies the 1273 +/- 1 Ma (2 sigma) Harp Dyke Swarm. In its lower parts, the Seal Lake Group includes conglomerates, quartzites, and red-bed sediments, and both the lower and middle parts are intruded by olivine gabbro sills. This magmatism was coeval with the emplacement of basaltic volcanic rocks in the middle and upper sections of the Seal Lake Group. Two different gabbro varieties yield baddeleyite and zircon U-Pb ages of 1250(-7)(+14) Ma and 1224(-5)(+6) Ma, respectively. The older age corresponds to the emplacement of a fine- to medium-grained voluminous olivine gabbro, whereas the younger age was obtained on a coarse-grained gabbro that intrudes the fine-grained gabbro. These ages place magmatism in the Seal Lake area in the ca. 1270-1220 Ma phase of extension-related magmatism of Laurentia and Baltica that was associated with voluminous mafic dyke swarms. Such extension-related mafic Mesoproterozoic magmatism occurs on both cratons during three distinct intervals at 1460-1420, 1270-1220, and 1180-1070 Ma, respectively, suggesting that Laurentia and Baltica formed a coherent unit until at least 1180 Ma.