AR-40 AR-39 GEOCHRONOLOGY OF THE WEST GREENLAND TERTIARY VOLCANIC PROVINCE/

Paleocene volcanic rocks in West Greenland and Baffin Island were amon g the first products of the Iceland mantle plume, forming part of a la rger igneous province that is now submerged beneath the northern Labra dor Sea. A Ar-40/Ar-39 dating study shows that volcanism commenced in West Greenland between 60.9 and 61.3 Ma and that similar to 80% of the Paleocene lava pile was erupted in 1 million years or less (weighted mean age of 60.5 +/- 0.4 Ma). Minimum estimates of magma production ra tes (1.3 x 10(-4) km(3) year(-1) km(-1)) are similar to the present Ic eland rift, except for the uppermost part of the Paleocene volcanic su ccession where the rate decreases to < 0.7 x 10(-4) km(3) year(-1) km( -1) (rift), The timing of onset of volcanism in West Greenland coincid es with the opening of the northern Labrador Sea and is also strikingl y similar to the age of the oldest Tertiary volcanic rocks from offsho re SE Greenland and the British-Irish province. This is interpreted as manifesting the impact and rapid (>1 m/year) lateral spreading of the Iceland plume head at the base of the Greenland lithosphere at simila r to 62 Ma. We suggest that the arrival, or at least a major increase in the flux, of the Iceland mantle plume beneath Greenland was a contr ibuting factor in the initiation of seafloor spreading in the northern Labrador Sea. Our study has also revealed a previously unrecognised E arly Eocene volcanic episode in West Greenland. This magmatism may be related to movement on the transform Ungava Fault System which transfe rred drifting from the Labrador Sea to Baffin Bay. A regional change i n plate kinematics at similar to 55 Ma, associated with the opening of the North Atlantic, would have caused net extension along parts of th is fault. This would have resulted in decompression and partial meltin g of the underlying asthenosphere, The source of the melts for the Eoc ene magmatism may have been remnants of still anomalously hot Iceland plume mantle which were left stranded beneath the West Greenland litho sphere in the Early Paleocene. (C) 1998 Elsevier Science B.V. All righ ts reserved.