Abstract

Volcanic and igneous rocks interbedded with Lower Cretaceous sedimentary rocks were penetrated in four exploratory oil wells in the Orpheus Graben, Scotian Shelf, offshore eastern Canada. The lower unit of the volcanogenic sequence is composed of basalt, as sills or feeder dikes or flows, a few metres to a few tens of metres thick. The upper unit consists of tens of metres of pyroclastic and volcaniclastic rocks in which volcanic tephra, including glass, is mixed with quartz-rich sediment interbedded with thin lava flows. These rocks show dips of as much as 36°, suggesting that they occur as volcanic cones.

The basalts are geochemically similar to within-plate oceanic alkali basalts, particularly in their rare-earth-element (REE) patterns and trace-element composition. Radiometric ages of 102 to 125 ± 5 Ma for the basalts are in good agreement with ages of interbedded sediments dated by palynomorphs as Aptian. The volcanic rocks are located on continental crust along the possible extension of two oceanic crustal features: the Newfoundland-Azores fracture zone and the Fogo Seamount Chain.

The Orpheus Graben alkaline volcanism may be an expression of a transform fault propagating into the continent as a result of rejuvenation during plate-motion changes in the late Lower Cretaceous, when continental spreading was initiated in the southern Atlantic and Labrador-Greenland regions. Mesozoic volcanism on the Atlantic continental margin was episodic, with a periodicity of 50–70 Ma. It is broadly time correlative with major episodes of intraplate oceanic volcanism in both the Atlantic and Pacific Oceans, indicating that the volcanism is mantle-driven.

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