BOSTON (AP) _ Shawmut Bank announced Monday the purchase of Dallas-based First Gibraltar Mortgage Corp. for more than $50 million in cash, its fourth major acquisition in little over a year.

''The acquisition makes us one of the 30 largest, with a total (mortgage loan) portfolio of $4.6 billion,'' said Shawmut spokesman Anthony J. Battista.

He said Shawmut would have the ability to offer more than $2 billion per year in new loans.

First Gibraltar has 20 offices in Arizona, California, Colorado, Georgia, Illinois, Minnesota, Texas, Wisconsin and Louisiana. It will operate as a subsidiary of Shawmut Bank.

In December 1985, Shawmut acquired North Central Mortgage Co. of Dayton, Ohio. It was renamed Shawmut Mortgage Co.

Prior to that, Shawmut purchased two Connecticut banks and North Central Financial Corp.

Shawmut has assets of $10 billion, making it one of the region's largest banks.