* Indexes return to positive territory, but well off highs
* Apple rallies, announces 7-for-1 stock split
* Caterpillar gains after results; Verizon, 3M fall
* Indexes up: Dow 0.2 pct, S&P 0.4 pct, Nasdaq 0.7 pct (Updates to midday trading)
By Ryan Vlastelica
NEW YORK, April 24 (Reuters) - U.S. stocks rose in a volatile session on Thursday as Apple and Caterpillar were the latest high-profile names to post bullish results, though gains were limited by a resurgence of concerns related to Ukraine.
Markets had opened sharply higher, with the Nasdaq initially climbing more than 1 percent before turning negative in the first half-hour of trading. While Wall Street subsequently recovered, with gains again concentrated in the Nasdaq, it remained well off its early highs.
Much of the volatility was driven by Russian Defense Minister Sergei Shoigu, who said Russia started military drills near the border with Ukraine.
The issue "is a lit stick of dynamite" with remote chances of a peaceful resolution, said Todd Schoenberger, managing partner at LandColt Capital in New York. "All those geopolitical concerns warrant instability and higher commodity prices, and both of those are things Wall Street hates."
Apple Inc, the most valuable U.S. company by market capitalization, rose 8.1 percent to $567.07 a day after posting revenue that far outpaced expectations, helped by strong iPhone sales. It also approved another $30 billion stock buyback plan, raised its dividend and authorized a seven-for-one stock split. Tech shares rose 1.2 percent as the best performing sector on the day.
"People have been critical of the cash on Apple's balance sheet, but now it has faced up to that criticism. It's doing all the right things, and I don't think a move over $600 would be out of place now," said Michael Binger, senior portfolio manager at Gradient Investments LLC in Minneapolis, which owns Apple.
Caterpillar Inc rose 3.1 percent to $106.60 after reporting better-than-expected earnings and raising its full-year profit view.
Limiting the Dow's advance, both Verizon Communications Inc and 3M Co fell after results missed expectations. Verizon dropped 2.4 percent to $46.29 and 3M fell 1.1 percent to $136.49.
The Dow Jones industrial average was up 31.54 points, or 0.19 percent, at 16,533.19. The Standard & Poor's 500 Index was up 6.82 points, or 0.36 percent, at 1,882.21. The Nasdaq Composite Index was up 28.17 points, or 0.68 percent, at 4,155.14. Both the S&P 500 and Nasdaq are up for their seventh session of the past eight.
Social media giant Facebook Inc posted a 72 percent jump in first-quarter revenue, lifted by its mobile advertising business. The stock fluctuated between positive and negative territory, last trading up 2.1 percent at $62.60.
Profits are seen rising 2.9 percent this quarter, down from the 6.5 percent growth rate estimated at the start of the year, according to Thomson Reuters data.
Zimmer Holdings Inc agreed to buy Biomet Inc in a deal valued at about $13.35 billion to broaden its portfolio of products that treat bone and joint-related disorders. Zimmer rose 12 percent to $102.80.
Sina Corp fell 4.6 percent to $50.75 after Chinese state media reported the company had been stripped of its online publication license after being targeted in a pornography crackdown.
A number of cloud-computing stocks, which have struggled lately but rose last week, opened higher and then quickly sold off. Workday Inc fell 5.1 percent while Salesforce.com was off 2 percent at $54.51. (Editing by Nick Zieminski)