The Loudoun County Board of Supervisors yesterday approved a 138-acre housing and shopping center development just west of Dulles International Airport, the second major rezoning in that area in two weeks.

Approval of the 301-home Arcola Glen project at the intersection of Rtes. 50 and 659 makes the Dulles area the probable center for growth in Loudon in the coming years. The rezoning calls for development to be phased in evenly over four years to its completed capacity of about 1,000 residents.

Two weeks ago the board approved a 196-acre, 1,700-resident community near Ashburn. A third development near Ryan is awaiting board action and a fourth project is soon scheduled to go to the county planning commission for initial consideration. Completion of all four would increase the county's population by about 5,000, roughly 10 per cent of the present population.

Yesterday's 5-to-3 vote for rezoning and December's 8-to-0 approval were the first significant developments to clear the County Board in more than five years.

Board members said the strong support of residents of the Arcola-Ryan-Ashburn area, which has been economically stagnant, and recent rulings in zoning cases that have overturned "unreasonable" rejections of rezoning applications have influenced the Board. A third factor has been the preference of board members from rural western Loudoun for keeping growth concentrated in the county's southeastern corner.

In another board action yesterday, William C. Crossman was re-elected chairman on a 5-to-3 vote over James F. Brownell after the board had dead-locked three times on 4-to-4 votes.

Brownell, the board's only Republican, had the support of the three Democrats from suburban eastern Loudoun.After the third deadlock and a 10-minute recess, George H. Yeager from Sterling switched his vote.

Yeager later said, "I felt leaving the county in a . . . quandary without a chairman would be harmful and division.

Yeager then was elected vice chairman on a 5-to-3 vote over Henry Stowers, vice chairman last year. Stowers was supported by Crossman, Ronald Blake and Stowers, all of whom had supported Crossman, while Yeager got the four votes that had gone to Brownell for chairman plus Frank Raflo of Leesburg.