Abstract
At the submerged margins of the North Atlantic, andesitic to dacitic
and
basaltic volcanic rocks
occur together. The silicic rocks were derived by processes requiring
the presence of continental crust
(crustal anatexis and/or contamination of mafic magmas) while the
majority of the basaltic lavas had little or
no contact with continental crust. We report 40Ar–39Ar
incremental heating ages for several dacitic and
basaltic rocks recovered from three offshore localities of the North
Atlantic Igneous Province. Dacitic lavas
and tuffs at the southeast Greenland margin and trachytic lavas in
the Scottish Hebrides erupted contemporaneously
with basaltic lavas at 62–61 Ma. In contrast, the silicic lavas from
the northern Rockall Trough
(offshore western Scotland) and the Vøring Plateau (offshore Norway)
erupted at ∼55 Ma followed shortly
by basaltic volcanism. At this time, silicic magmatism at the southeast
Greenland margin had ceased and
only oceanic basalts were erupted. Similarly, ∼55 Ma lavas on the
southwest Rockall Plateau are wholly
basaltic. The compositions of all of the dated silicic volcanic rocks are
consistent with derivation from partial
melting of either continental crust or sediments. The heat necessary
for partial melting appears to have
been provided by basaltic magmas. Therefore, the existence of the
silicic rocks indicates the presence of
continental 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 sides of
the present North Atlantic margins were characterized by extensional
environments within continental crust
that were restrictive to the passage of mafic magmas. By 55 Ma, at the
time of continental breakup, the proximal
margins at southeast Greenland and the Rockall Plateau were devoid of
continental crust. But the presence
of 55 Ma silicic magmatism on the eastern North Atlantic margin can
be attributed to a broader zone of
magmatism and sediment-filled Mesozoic rift basins.