Results of (super 40) Ar- (super 39) Ar Ar dating constrain the age of the submerged volcanic succession, part of the seaward-dipping reflector sequence of the Southeast Greenland volcanic rifted margin, recovered during Leg 163. At the 63 degrees N drilling transect, the fully normally magnetized volcanic units at Holes 989B (Unit 1) and 990A (Units 1 and 2) are dated at 57.1+ or -1.3 Ma and 55.6+ or -0.6 Ma, respectively. This correlates with a common magnetochron, C25n. The underlying, reversely magnetized lavas at Hole 990A (Units 3-13) yield an average age of 55.8+ or -0.7 Ma and may correlate with C25r. The argon data, however, are also consistent with eruption of the lavas at Site 990 during the very earliest portion of C24. If so, the normally polarized units have to be correlated to a cryptochron (e.g., C24r-11 at approximately 55.57 Ma). The lavas at Holes 989B and 990A have typical oceanic compositions, implying that final plate separation between Greenland and northwest Europe took place at approximately 56 Ma. The age for Hole 989B lava is younger than expected from the seismic interpretations, posing questions about the structural evolution of the margin. An age of 49.6+ or -0.2 Ma for the basaltic lava at Site 988 ( approximately 66 degrees N) points to the importance of postbreakup tholeiitic magmatism at the rifted margin. Together with results from Leg 152, a virtually complete time frame for approximately 12 m.y. of pre-, syn-, and postbreakup volcanism during rifted margin evolution in Southeast Greenland can now be assembled. This time frame includes continental type volcanism at approximately 61-60 Ma, synbreakup volcanism beginning at approximately 57 Ma, and postbreakup volcanism at approximately 49.6 Ma. These discrete time windows coincide with distinct periods of tholeiitic magmatism from the onshore East Greenland Tertiary Igneous Province and is consistent with discrete mantle-melting events triggered by plume arrival ( approximately 61-60 Ma) under central Greenland, continental breakup ( approximately 57-54 Ma), and passage of the plume axis beneath the East Greenland rifted margin after breakup ( approximately 50-49 Ma), respectively.