AR-40-AR-39 GEOCHRONOLOGY OF SILICIC AND BASIC VOLCANIC-ROCKS ON THE MARGINS OF THE NORTH-ATLANTIC

At the submerged margins of the North Atlantic, andesitic to dacitic a nd basaltic volcanic rocks occur together. The silicic rocks were deri ved by processes requiring the presence of continental crust (crustal anatexis and/or contamination of mafic magmas) while the majority of t he basaltic lavas had little or no contact with continental crust. We report Ar-40-Ar-39 incremental heating ages for several dacitic and ba saltic rocks recovered from three offshore localities of the North Atl antic Igneous Province. Dacitic lavas and tuffs at the southeast Green land margin and trachytic lavas in the Scottish Hebrides erupted conte mporaneously with basaltic lavas at 62-61 Ma. In contrast, the silicic lavas from the northern Rockall Trough (offshore western Scotland) an d the Voring Plateau (offshore Norway) erupted at similar to 55 Ma fol lowed shortly by basaltic volcanism. At this time, silicic magmatism a t the southeast Greenland margin had ceased and only oceanic basalts w ere erupted. Similarly, similar to 55 Ma lavas on the southwest Rockal l Plateau an wholly basaltic. The compositions of all of the dated sil icic volcanic rocks are consistent with derivation from partial meltin g of either continental crust or sediments. The heat necessary for par tial melting appears to have been provided by basaltic magmas. Therefo re, the existence of the silicic rocks indicates the presence of conti nental crust as well as a stable tectonic environment that allowed the stagnation and pooling of basaltic melts within the crust. With this in mind, it is apparent that at 62-60 Ma, both western and eastern sid es of the present North Atlantic margins were characterized by extensi onal environments within continental crust that were restrictive to th e passage of mafic magmas. By 55 Ma, at the time of continental breaku p, the proximal margins at southeast Greenland and the Rockall Plateau were devoid of continental crust. But the presence of 55 Ma silicic m agmatism on the eastern North Atlantic margin can be attributed to a b roader zone of magmatism and sediment-filled Mesozoic rift basins.