A brutal new year selloff in oil markets quickened on Monday, with prices plunging 6 percent to new 12-year lows as further ructions in the Chinese stock market threatened to knock crude as low as $20 a barrel. The latest catalyst was a further 5 percent decline in China’s blue-chip stocks and a surge in overnight interest rates for the yuan outside of China to nearly 40 percent, their highest since the launch of the offshore market.