The International Monetary Fund yesterday approved a $1.4 billion 1985 financing package for cash-starved Argentina, bringing to a close the first phase of the Latin American debt crisis that erupted in August 1982.

The IMF action came after a group of bank lenders pledged to lend Argentina at least 90 percent of the $4.2 billion the country's financial lenders agreed Argentina needs to borrow next year.

IMF Managing Director Jacques de Larosiere had said the banks had to commmit at least 90 percent of the $4.2 billion before he would ask the international agency's executive board to approve its part of the plan.

The IMF agreement is critical -- not because of the amount of money involved, but because it requires Argentina to take steps to reduce its explosive 700 percent inflation rate and to lessen its need to borrow abroad in future years. The IMF reached a tentative pact with Argentina in late September, then waited for the banks to do their part before ratifying the agreement yesterday.

The banks also agreed to stretch out for 10 to 12 years repayment of the $13.4 billion in Argentine loans that either have come due or will be due before the end of 1985.

Argentina's first agreement with bankers and the IMF comes nearly two years after Mexico and Brazil came to terms with their creditors. Mexico and Brazil, with about $100 billion each in foreign loans, are the region's biggest competitors. Argentina's $45 billion in foreign debts, $25 billion of which is owed to banks, makes it the region's third biggest borrower.

Nearly all Latin American nations have had difficulty repaying their foreign debts since mid-1982. The countries fell victim to their own excessive borrowing as well as to economic developments in industrial countries that raised interest rates and cut the prices of and demand for the commodities the debtor nations export to earn dollars to pay their loans.

Mexico, Brazil and most other debtor nations already have taken steps to put their economies in order. They cut subsidies and other spending to reduce their budget deficits and the need to borrow. They held down imports and devalued their currencies in order to increase the foreign exchange they earn and that is needed to pay their debts. Both Mexico and Brazil have said they will not need to borrow new money next year and are negotiating pacts that will restructure all their bank debts. The pacts also include far better terms than the countries negotiated in 1982 and 1983.

But Argentina -- in the last year of a military dictatorship and the first year of a democracy -- resisted taking those steps, fearing the domestic consequences of the recession that was expected to follow. Argentine officials said they feared a recession would threaten the new democracy. They also worried about the consequences of seeming to bow to the IMF in a country that is intensely nationalistic.

But the Argentine inflation rate that already was the highest in the world worsened throughout most of 1984, convincing the new government of Raul Alfonsin that it had to act.

Argentina reached a tentative agreement with the IMF last September. The agreement calls for Argentina to hold down wage increases, devalue its peso and take other steps to reduce the inflation rate and hold down its need to borrow. The tentative agreement also called for $1.4 billion in IMF funds and clears the way for bankers to begin negotiating with the debtor country.

After the agreement was announced about 6:30 p.m., acting Treasury Secretary R.T. McNamar issued a statement saying, "This bridge loan is made to support President Alfonsin's economic program, which was endorsed by the managing director of the IMF and is designed to return the Argentine economy to a path of more sustainable, less inflationary real economic growth."

Bankers were angered with Argentina because of its nearly two years of foot-dragging. The country was far behind on its principal payment and, more important to banks, disasterously behind on interest payments. The interest arrearages not only cut into bank earnings at a time when financial institutions were strained, but also forced U.S. banks to put most Argentine loans on special-problem lists.

But the banks finally agreed to make Argentina more money, in part because the country promised to clear up most of its past-due payments by Dec. 31 and in part to protect the sizeable loans they have already made to the country. Argentina has said it will pay about $850 million of its $1.2 billion in overdue interest payments before New Year's so banks can count the payments in their 1984 earnings.

Argentina's key bank lenders -- led by Citibank executive William A. Rhodes -- failed to meet their self-imposed Christmas deadline for getting enough commitments from Argentina's 320 bank lenders. But enough banks were on board yesterday morning to enable De Larosiere to take the tentative pact to the IMF board on schedule.

Had the IMF had to wait until after New Year's day, the agency would not have been able to lend the debtor nation $1.4 billion. On Jan. 1, all countries will have their borrowing rights reduced as a result of changes pushed through this year by the United States.

In Argentina's case, the reduction would have been about $100 million.