Second-half goals by Chris Tomek, Cathy Moon and Dana Hedin propelled George Mason past No. 1-ranked, previously unbeaten Massachusetts, 3-0, in the NCAA semifinals before 4,000 yesterday at George Mason.

North Carolina, seeded No. 2, will meet the Patriots at 1:30 today at George Mason for the championship. UNC defeated Colorado College, 3-2, as Marcia McDermott from McLean High School scored two goals, including the game-winner.

Two years ago, the Tar Heels (18-1-1 this year) defeated George Mason, 4-0, for the NCAA title. The teams tied, 3-3, in the 1985 season opener.

Coach Hank Leung's George Mason team (17-2-1) tested Massachusetts several times in the first half, began taking advantage of UMass' offensive-minded fullbacks by using a counterattacking style and at last overwhelmed freshman goalie Jan Holland on the resulting mismatches.

"This is the biggest upset in the history of our school's program," said Leung, who started women's varsity soccer at George Mason four years ago. "We reached the finals two years ago, but we weren't ready. This year we are."

Until yesterday Massachusetts had allowed two goals all season and Holland had a 0.07 goals-against average.

Mason determined the tempo from the outset of the second half, but did not take control until Tomek outraced Holland for a deflected shot and made a five-foot shot into the left corner of the net at 56:40. Eight minutes later, Moon scored on an assist from Lisa Gmitter. Hedin scored at 79:30 on a steal, a move between two defenders and a 10-foot shot into the right corner.

Outstanding in the Patriots' defense were Sheryl Walters, Betsy Drambour, Kim Crabbe and Sis Koslinen, who allowed just six shots to reach goalie Kim Maslin. It was the ninth shutout for Maslin.

"Four years ago many of us helped start this program and dreamed that we'd be champions one day," said Mason captain Pam Baughman. "And tomorrow's our last chance and we're going to make the most of it."