Interior Secretary Cecil D. Andrus, in what he said was "one of my toughest calls," yesterday approved construction on public lands of the massive Harry Allen powerplant in southern Nevada, but postponed a decision on the companion Warner Valley plant in southern Utah. The decision disappointed all sides.

Andrus' decision is a half-step forward for the 2,500-megawatt Allen-Warner Valley Energy System, a proposed $4 billion complex of strip mines, artificial reservoirs, water and coal slurry pipelines and transmission towers in the heart of the so-called "Golden Circle" of national parks. More than 80 percent of its power would go to Southern California.

Ron rudolph of the San Francisco office of Friends of the Earth said the decision was "unfortunate" and probably means the overall project will go forward, since the incoming Republican administration is likely to approve the Warner unit eventually. "It is a needless blemish on the end of Andrus' term," Rudolph said.

John W. Arlidge, special projects manager of the Nevada Power Co., said the Allen plant approval was "a first step" and added that the consortium behind the project would try to correct any problems with the other plant by March. "We hope to be through the approval process by the end of summer," he said.

That process still involves approvals from the utility boards of California, Nevada and Utah, and Rudolph said environmentalists will continue to fight "any way we can." The opposing groups succeeded last month in persuading Andrus to ban strip mining for the project in a portion of the Alton coal fields near scenic Bryce mountain National Park in Utah, but that action still left enough mining options available to keep the project going.

Andrus said his split decision rested on "potentially serious environmental problems associated with the project which have not yet been resolved." The Environmental Protection Agency tentatively rejected the Warner plant last year on grounds that it would cause too much deterioration in air quality, but a final verdict is not expected until next month.

In addition, two endangered fish species live in the Virgin River, which would be a water source for the plant, and the Fish and Wildlife Service is discussing that with the area water conservation authorities.