基金financially supported by Key Laboratory of Hexi Corridor Resources Utilization of Gansu Universities(No.XZ1011 )the President's Funds of Hexi University(No.XZ-2009-9)the National Natural Science Foundation of China(No.21262010)
文摘Received 20 November 2012 Received in revised form 13 December 2012 Accepted 21 December 2012 Available online 29 January 2013