UPDATE 1-In debut, Shanghai crude oil futures jump more than 6 pct

* September turnover eclipses Brent June volume

Retail, institutional trading driving volume in early trade

* Prices jump as Brent pierces $70/barrel (Adds detail throughout)

By Josephine Mason and Meng Meng

BEIJING, March 26 (Reuters) - Shanghai crude oil futures opened up more than 6 percent on the Monday debut of China's new oil benchmark as trading by mom-and-pop and institutional investors eclipsed turnover in the most-active contract of established rival Brent.

Around 12 million barrels of Shanghai's most-active September contract changed hands in the first 55 minutes of trade, more than the most-active contract for Brent. Trading of Brent futures on the Intercontinental Exchange (ICE), however, is usually most active after London opens around 0700 GMT.

The launch of China's yuan-denominated oil futures marked the culmination of a decade-long push by the Shanghai Futures Exchange (ShFE) to give the world's largest energy consumer more power in pricing crude sold to Asia.

The most-active September contract opened at 440.4 yuan ($69.78) per barrel from a reference point of 416 yuan, jumping as high as 447.1 yuan ($70.85) in the first few minutes.

That came after Brent futures for May delivery opened above $70 per barrel for the first time since January, boosted by expectations that OPEC-leader Saudi Arabia may extend supply cuts into 2019, as well as concerns that the United States may re-introduce sanctions against Iran.

At 9:56 a.m. (0256 GMT), Shanghai prices were up 4.4 percent at 434 yuan, with 24,132 lots traded. Chinese exchanges count each way of the trade - the buy and sell - as two lots.

That means total oil changing hands was 12,066 lots, equal to 12.06 million barrels.

Brent volume in June was 10,500 lots, equivalent to 10.5 million barrels of oil.

Liquidity was driven by speculative retail and institutional investors, said Chen Tong, Shanghai-based senior crude analyst at First Futures.

"In the short term, we believe price fluctuations will reflect domestic crude oil supply and demand. In the long run, yuan crude price will mirror the moves of Brent," he said.

For an EXPLAINER on market concerns, click on

For a FACTBOX on contract details, specification, click on

($1 = 6.3109 Chinese yuan)

(Reporting by Josephine Mason and Meng Meng; Editing by Richard Pullin and Tom Hogue)