The Reagan administration's decision to impose economic sanctions on Argentina is largely a symbolic gesture designed to demonstrate U.S. support for Great Britain in its dispute over the Falkland Islands, according to bankers and administration officials.

"Its impact on Argentina will be relatively minor," said W. Denis Wright, senior vice president of Continental Illinois National Bank.

The major step the administration took was to cut off further credits and loan guarantees from the Export-Import Bank to finance U.S. exports to that country. The bank has commitments of about $1.2 billion to Argentina. Of that amount, about $895 million would be direct disbursements from the bank, while the rest is in the form of guarantees and insurance. Less than $200 million of the $895 million actually has been disbursed, a bank spokesman said.

The $200 million figure will grow somewhat because the bank will disburse money to finance products that already have been shipped. All other commitments will be suspended, however.

Continental's Wright said Argentina is not in a position to import much anymore, at least until the current crisis ends, because of its inability to raise funds from the world's banks. Bankers estimate that Argentina has about $12 billion in loan repayments due this year that it must refinance.

Wright said that Argentina could replace most of its imports from the United States by turning to Brazil.

Senior U.S. officials agreed yesterday that the nation has little economic leverage over Argentina but warned that the United States has stronger weapons in its arsenal that it could use. According to statistics published by the International Monetary Fund, the United States shipped $2.6 billion of goods to Argentina in 1980, while Argentina sold $690 million to the United States.

Besides the Export-Import Bank restrictions, Secretary of State Alexander Haig also announced that the government will halt any arms sales to Argentina and cancel any further loan guarantees under a one-year, $2 million Commodity Credit Corp. program to support lumber exports to Argentina.

Arms shipments to Argentina have been embargoed since 1978, although the United States continued to ship military goods if the commitments were entered into before Sept. 30, 1978. Yesterday Haig suspended those shipments as well as shipments to nongovernment entities, which had continued.

The administration said the arms cut-off affects about $5.9 million of sales--both by the government and private firms.

The stiffest sanctions Argentina faces were imposed by the European Economic Community, which essentially ended trade with the South American country as a show of support for Great Britain.

The Export-Import Bank said that the biggest credit suspension was a $550 million commitment to help build a hydroelectric project on the Argentina-Paraguay border. Among major suppliers to the project are Ford Motor Co., Caterpillar Tractor Co. and Allis Chalmers Corp.

But one banker, who asked not to be identified, said that the Export-Import Bank restrictions presumably would be temporary and would be lifted by the administration if and when the confrontation between Britain and Argentina ends.

Argentina is strapped for cash--in large part because London essentially has frozen Argentine deposits in Great Britain, and other multinational banks are reluctant to make new loans to the country until the crisis ends. However it continues to make payments on its outstanding bank loans, bankers said.