Geology and Geochemistry of Middle Jurassic and Early Cretaceous Igneous Rocks On the eastern North American Continental Shelf

Abstract

Late Middle Jurassic and Early Cretaceous mafic dykes, sills, flows and locally associated volcaniclastics occur from the Baltimore Canyon Trough northwards to the Labrador Shelf. The late Middle Jurassic activity was of a short duration at about 140 m.y. and is documented here from the Georges Bank. The Cretaceous igneous activity, which is more regionally widespread, continued for about 40 m.y. (95-135 m.y.). All the rocks analysed are alkali basalts and related mafic rocks. Rocks from the Baltimore Canyon, Georges Bank and New England Seamounts show marked enrichment in light REE and many incompatible trace elements, and appear to have been derived by a small amount of partial melting of enriched mantle of the type associated with mantle plumes. In contrast, rocks from the Scotian Shelf and Grand Banks show slightly convex REE spectra and less enrichment in incompatible elements and appear to be derived from normal (non-enriched) mantle. Limited data for the Newfoundland Seamounts suggests that they are associated with a slightly enriched mantle source. The regional geochemical differences noted between the Grand Banks - Scotian Shelf igneous province and the Georges Bank - Baltimore Canyon igneous province may reflect basic differences in mantle composition and evolutionary history. This may be related to; the development of a mantle plume beneath the Georges Bank - Baltimore Canyon trough province. The Cretaceous volcanism coincides with reactivation and formation of fracture zones as a result of changes in plate stresses culminating in separation of the Grand Banks and Iberia and initiation of rifting in the Labrador Sea. The late Middle Jurassic igneous activity on Georges Bank led to the construction of several volcanic cones. The regional distribution of the volcanic ash suggests prevailing offshore winds, blowing to the east. The occurrence of both Middle Jurassic and Early Cretaceous buried volcanoes under the upper continental slope off Georges Bank sheds new light on the origin of the New England Seamount chain. An initial shear failure, which probably penetrated the uoper mantle focussed on regional stresses to develop a propagating fracture zone which became the loci of igneous activity at its propagating tip. After decoupling of the continental plates and the initiation of seafloor spreading in the Labrador Sea (83-92 m.y.), the igneous activity ceased along the eastern North American margin and at the propagating tip of the New England Seamount fracture zone.