The lame-duck 96th Congress presented a windfall to central Virginia preservationists, essentially reversing a federal judge's recent ruling on controversial historic acreage.

The legislation, signed last week by President Carter, reinstates the historic-landmark status previously assigned by the Interior Department to the Green Springs area -- 14,000 rolling acres about 90 miles south of Washington in Louisa County, where prospectors believe rich mineral deposits may lie.

The area is one of the largest landmark designations ever made by the Interior Department, which singled it out, a department official said yesterday, for its relatively unspoiled representation of 19th-century rural and farm life.

Preservationists and strip-mining interests in the county have been at loggerheads for 11 years over competing claims for the area's historic virtures and what miners believe are rich veins of vermiculite -- used in kitty litter and insulation -- beneath the surface. The congressional action won't halt the present mining in the area, but officials said it should effectively thwart expansion of the mines into areas that were declared unprotected last summer by a federal judge in Richmond.

District Court Judge Robert R. Merhige last August threw out historic status for the area -- which includes nearly two dozens historic homes and buildings -- on grounds that the Interior Department had not established clear standards for what constitutes a landmark. Merhige criticized the historic-designation process as "vague and open-ended."

Rep. John F. Seiberling (D-Ohio) and Sen. Dale L. Bumpers (D-Ark.), who authored the legislation, took Merhige's questions to heart, congressional staffers said. Their new law addressed the major criticisms the judge made and requires Department to give more advance notice of proposed historic designations.

A crucial element of some designations is the "preservation easement" created when landowners surrender their property to federal protection against untoward development. In his ruling, Merhige had invalidated easements existing on nearly 8,000 acres of Green Springs territory. The congressional action reinstates them.