K-Ar dating of granitic rocks in northeastern Washington and northern Idaho indicates four distinct periods of intrusion at about 200+ m.y., 170 m.y., 93 to 101 m.y., and 45 to 51 m.y. The two oldest events are each represented by only one pluton in the region. Plutons of the 93- to 101-m.y. event are numerous in the eastern and western parts of the region and may have occupied much of the intervening area before the youngest event. Except for a group of unrooted plutons in the upper plate of a thrust fault, no plutonic rocks between the west edge of the Pend Oreille River valley and the Purcell Trench south of lat 48° 15′ N. yielded an age outside the 45- to 51-m.y. bracket. Westward from the Pend Oreille River valley, the plutonic rocks yield successively older ages over a zone 8 to 25 km wide. All coexisting mineral pairs from rocks within this zone yield discordant ages. West of the zone, mineral pairs yield concordant ages in the 93- to 101-m.y. range. The apparent ages were contoured over about 15,000 km2, and the position and trend of discordant ages have been delineated.
East of the Purcell Trench, a partly preserved mirror image of the Pend Oreille River zone of discordance is disrupted by faults in the trench and in places appears to have been removed. In the vicinity of the Purcell Trench, the apparent ages suggest a complex thermal history probably complicated by faults concealed in the trench. The apparent ages can be contoured at least two different ways, one interpretation assuming that the contours are disrupted by a large fault or fault system in the trench, the other assuming either that no faults exist or that the faults predate the 45 to 51 m.y. intrusions. The contours can also be drawn either being offset by the Hope fault or crossing the fault without interruption. The distribution of granitic rock outcrops around the Purcell Trench and Hope fault is such that a unique solution cannot be determined for either area.