* Momentum, biotech stocks take another plunge
* S&P 500, Nasdaq post worst weekly decline since June 2012
* Dow down 0.9 pct; S&P 500 off 1 pct; Nasdaq down 1.3 pct (Updates to close)
By Angela Moon
NEW YORK, April 11 U.S. stocks slid in a volatile session on Friday, with the Nasdaq closing below the 4,000 mark for the first time since early February.
Selling accelerated late in the afternoon, with the biotech and other momentum stocks again leading the Nasdaq sharply lower. JPMorgan's disappointing earnings also gave investors a reason to sell some bank stocks.
For the week, the S&P 500 fell 2.6 percent and the Nasdaq lost 3.1 percent, the biggest weekly decline for both indexes since June 2012.
"Today's decline is what we've been seeing all week. The weakness in the biotech and momentum names is getting investors worried about where the market is headed in the near-term, eventually triggering a selloff in everything," said Robert Pavlik, chief market strategist at Banyan Partners in New York.
"Our long-term outlook on the market hasn't changed because if you understand why the market is selling off, you know it's not rational, that it doesn't make sense," he added.
The Dow Jones industrial average fell 143.47 points or 0.89 percent, to end at 16,026.75. The S&P 500 lost 17.39 points or 0.95 percent, to finish at 1,815.69. The Nasdaq Composite dropped 54.372 points or 1.34 percent, to close at 3,999.734.
The Nasdaq Composite fell through 4,000 for the first time since early February and many one-time market darlings are now down substantially from records reached only six or seven weeks ago.
JPMorgan Chase & Co shares fell 3.7 percent to close at $55.30. The stock was the biggest drag on the S&P 500 after the bank reported a far weaker-than-expected quarterly profit as revenue from securities trading fell.
The S&P financial index dropped 1.2 percent. It was the S&P 500's worst-performing sector.
The Nasdaq biotech index fell 2.8 percent after rising as much as 1 percent earlier. The Global X social media index, which includes Facebook and LinkedIn , slid 2.3 percent. Facebook shares fell 1.1 percent to $58.53. LinkedIn shares lost 2.5 percent to end at $165.78.
In contrast to the day's sharp downturn, shares of Wells Fargo & Co rose 0.8 percent to $48.08 after the biggest mortgage lender in the United States reported a 14 percent increase in first-quarter net profit.
Shares of Herbalife sold off late in the day after the Financial Times reported that the Department of Justice and the FBI had launched a probe into the company. The stock tumbled 14 percent to close at $51.48.
Even with the recent declines, investors appear committed to equities. Investors in U.S.-based funds poured $8.9 billion into stock funds in the week ended April 9, data from Thomson Reuters' Lipper service showed on Thursday.
The latest economic data showed that consumers felt more optimistic as April got under way. The Thomson Reuters/University of Michigan's preliminary April reading on the overall index of consumer sentiment came in at 82.6, the highest since July, as both current conditions and expectations brightened.
The seasonally adjusted producer price index for final demand increased 0.5 percent last month, the largest increase in nine months, pointing to some pockets of inflation at the factory gate.
Trading volume was around 7.3 billion shares on U.S. exchanges, above the 6.9 billion average so far this month, according to data from BATS Global Markets. (Editing by Jan Paschal)