China’s stock markets plunged on Thursday, with indexes dropping over 6 percent in record high turnover as investors rushed to sell after more brokers tightened margin trading requirements for clients and the central bank drained money market liquidity. The CSI300 index and the Shanghai Composite Index both slumped in late afternoon trade, ending down 6.7 percent and 6.5 percent, respectively, their worst day since January 19 when markets fell over 7 percent on an earlier crackdown on margin trading.