The Rockies swept the two-game series and earned their fourth consecutive victory.
Trevor Story had a hit and a walk, scored two runs and stole a base for Colorado.
The A's got on the board in the first inning when Chapman hit a two-out, full-count fastball over the center field wall for his first home run of the season.
Marquez (1-1) settled down after that, and he wound up allowing just four hits and one walk.
Colorado tied it in the second but missed a chance for more against A's starter Frankie Montas. After Daniel Murphy walked and McMahon singled to lead off the inning, Sam Hilliard struck out before Garrett Hampson walked to load the bases. Tony Wolters hit a ball to the wall in right that Stephen Piscotty couldn't corral, allowing Murphy to score from third.
But Montas (0-1) then got David Dahl to pop up to second and Story to ground out to keep the score 1-1. Montas allowed two runs on five hits and two walks in five innings. He struck out three.
The Rockies added a run in the fourth when McMahon led off with a single, went to third on Hilliard's single to right and scored on Hampson's sacrifice fly to left to make it 2-1.
Oakland had a chance to tie in the bottom of the frame when Matt Olson reached on a one-out walk before a double by Grossman put runners on second and third with two outs. Piscotty hit a sharp grounder to third, and Nolan Arenado's throw home beat Olson. The A's challenged the tag play, but the call was upheld after a review.
Colorado added another run in the eighth on Blackmon's RBI double. The Rockies padded the lead in the ninth when Blackmon beat out a grounder back to the pitcher with two outs, driving in Hampson from third while Story hustled home from second to make it a four-run game.
-Field Level Media