Pierre Trudeau, in his first speech after an overwhelming election victory last night, hinted broadly today that he intends to return to a policy of greater independence from the United States.

Like a born-again economic nationalist, Trudeau raised the prospect of expanded Canadian ownership and control over an economy dominated by $50 billion in U.S. investments.

Trudeau has said that before. He promised in 1974 to achieve 50 percent Canadian ownership of this country's oil and gas industry by 1980. Now the target is 1990.

In the 1970s, Trudeau pursued the so-called "third option" policy when he unsuccessfully sought to lessen Canada's dependence on the United States by developing ties with Third World, European and Soviet-bloc countries.

The difference this time, however, is that the 60-year-old Liberal leader has bounced back from retirement with a mandate for a fourth term won overwhelmingly on his own conditions.

A preview of the direction Canada's foreign policies under Trudeau, and especially the country's relations with the United States, will come when he decides whether to follow President Carter's plea that Western countries boycott the Moscow summer Olympics to show displeasure with the Soviet invasion of Afghanistan.

The defeated Conservative government of prime minister Joe Clark had announced that Canada would boycott the games if the Soviet troops are not withdrawn by Wednesday, the deadline set by Carter.

Trudeau, however, has said he would favor the boycott only if "there is a massive participation" of Third World as well as Western countries. His spokesman said today that Trudeau would have no comment on Wednesday's deadline.

With 146 of Parliament's 282 seats, Trudeau is beyond the reach of opposition parties. The Progressive Conservatives won 103 seats, the socialist New Democratic Party 32 seats, while the Social Credit Party won none. It had five seats in the last Parliament. One seat remained vacant because of the death of a candidate.

The popular vote shows how heavily Canadians favored the Liberals. Of 10.8 million ballots, the Liberals pulled 46.2 percent, the Conservatives 31.7 percent and the socialists 19 percent, with splinter parties getting 3.1 percent.

Trudeau avoided making promises to the electorate or his own party -- except to prevent the excise tax of 18 cents a gallon on gasoline that Clark had promoted.

By announcing an intention to quit politics after this term, Trudeau also made himself practically invulnerable to the pressure experienced by politicians who worry about the next election.

As a result, Trudeau is seen here as having won a political blank check on which to write his program. Just how he intends to use it is unclear.

Whatever Trudeau's decision on the Olympic boycott, relations between Ottawa and Washington are likely to change in style if not in substance.

Obviously trying to distance himself from what was perceived here as the overwhelmingly pro-American Clark government, Trudeau talked about "many friends" Canada has throughout the world -- "friends in NATO and other such alliances."

"But we do not forget, nor does the world, that Canada is geographically situtated between the two superpowers and that Canada, like the rest of the world, is very much interested in the preservation of peace between these two superpowers," he said.

Close associates say Trudeau is disturbed by the deterioration of Soviet-American relations and is likely to go to Washington soon for talks with President Carter.

Trudeau made it clear that in domestic affairs he will interpret his victory as a mandate to build his kind of strong central government. This means battles of Quebec's upcoming sovereignty referendum and Alberta's drive for higher energy prices.

The conflict with Alberta seems inevitable because Trudeau is commited to keeping domestic oil prices below world levels. The Clark government agreed to allow the oil-rich Western province to raise the price of domestic crude to 85 percent of the world price over the next four years. This will now to be renegotiated.

Despite the overwhelming Liberal victory, the party won only two seats west of Ontario, a fact that is likely to lead to a sense of alienation among the resources-rich western provinces. They were heavily represented in the nine-month Clark government.

The impact of Trudeau's victory on French-speaking Quebec also is unclear. The Liberals, always strong there, won 73 of the province's 74 seats yesterday. Quebec is to hold a referendum in June on whether the province should become an independent nation in economic association with the rest of Canada.

When Trudeau was defeated last May, after 11 years as prime minister, the election left Canada split along linguistic lines. Yesterday English-speaking Ontario, which accounts for one-third of the country's population and half of its industrial output, swung back behind Trudeau, presumably endorsing his concept of Canadian unity.

On the same state where Trudeau conceded defeatt 273 days ago, he vowed last night to fight for a Canada that will remain "more than the sum of its parts. I ask you that we put the country before ourselves or our region," he said. The strength of his mandate now is greater than during his previous years in power.

Just how long Trudeau intends to stay as prime minister he has not declared. "I have promises to keep," he said in closing his speech, with the familiar lines from Robert Frost, "and miles to go before I sleep, and miles to go before I sleep."