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 127 3 +/- 1 Ma (2 sigma) Harp Dyke Swarm. In its lower parts, the Seal Lak e 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 r ocks in the middle and upper sections of the Seal Lake Group. Two diff erent gabbro varieties yield baddeleyite and zircon U-Pb ages of 1250( -7)(+14) Ma and 1224(-5)(+6) Ma, respectively. The older age correspon ds to the emplacement of a fine- to medium-grained voluminous olivine gabbro, whereas the younger age was obtained on a coarse-grained gabbr o 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 voluminou s mafic dyke swarms. Such extension-related mafic Mesoproterozoic magm atism occurs on both cratons during three distinct intervals at 1460-1 420, 1270-1220, and 1180-1070 Ma, respectively, suggesting that Lauren tia and Baltica formed a coherent unit until at least 1180 Ma.