China shares tumbled Friday, even as Beijing moves swiftly to try to plug losses, with the smaller Shenzhen index suffering its worst week since December 1996. The Shanghai Composite, which ended down 5.8%, lost 12.1% this week and more than a quarter of its value since a high on June 12. The Shenzhen Composite fell 5.3% while the ChiNext board, composed of small-cap stocks, lost 1.7%. Both are down more than a third from peaks last month, with weekly losses of 16.2% and 10.8%, respectively. Chinese officials over the past week have dug deep for measures to stop massive stock selloffs. Still, the array of attempts—from lifting restrictions on investing with borrowed funds to cutting interest rates —so far have failed to encourage investors to buy, even though China’s main stock index has nearly doubled since last year. On Friday, China’s securities regulator said it would slow the pace […]