NEW YORK — Rates on fixed mortgages dropped to their lowest levels in decades this week after the Federal Reserve unveiled a massive bond-buying program to help spur economic growth.

Mortgage buyer Freddie Mac says the average rate for 30-year fixed loans fell to 4.17 percent from 4.24 percent last week. That’s the lowest on records dating back to 1971.

The average rate on 15-year fixed loans fell to 3.57 percent from 3.63 percent. That’s the lowest since the survey began in 1991.

The Fed detailed plans last week to buy $600 billion in Treasury bonds. The extra demand means Treasurys will produce lower yields for investors. Mortgage rates tend to track those yields.