General Motors of Canada and the United Auto Workers made a major breakthrough in contract talks yesterday that could lead to a quick settlement in the week-old strike that has caused layoffs of more than 18,000 workers in the United States, a union official said.

"The impasse has been broken," Canadian UAW director Robert White said following a 4 1/2-hour meeting with union negotiators. He said the two sides "have found a key to unlock some fundamental issues."

The strike that began Oct. 17 has threatened further layoffs and cutbacks at companies in the United States that supply 13 plants in Ontario and Quebec.

White said the breakthrough stemmed from a private meeting with GM Canada chief negotiator Rod Andrew earlier in the day.

White said the union had reached a "formula" with the auto maker, but he would not be more specific because he said talks remained at a crucial stage. GM spokesmen were unavailable for comment.

In a separate labor issue involving the UAW and Canadians, Mack Trucks Inc. announced that it has laid off 416 workers at its production plant in Oakville, Ontario, because it is unable to receive parts from its facility in Hagerstown, Md., one of four U.S. plants struck Sunday by 9,200 UAW members after their old contract expired.

Meanwhile, representatives of Mack and the UAW resumed negotiations yesterday at an undisclosed location in an attempt to end the walkouts at Hagerstown, Macungie, Pa., Bridegwater, N.J., and Allentown, Pa., where Mack is based.