���ຣ�����ӵ��������ĸ�ӿ�����ҽ��� U-Pb ���ѧ�͵���ѧ�����о����������: �����ĸ�ӿ�������ʯ U-Pb ����Ϊ32. 39 ��34. 06 Ma����Ȩƽ������Ϊ (33. 09 ±0. 26) Ma���γ�ʱ��Ϊ������; ��ʯ���ڸ߹�߼�׼����--�����ʼ�����ϵ�С�ϡ��Ԫ�����ģʽͼ�������ͣ�����ϡ��Ԫ�ط������� ��(La /Yb) N =19. 96 ��34. 00��ƽ��ֵ 26. 99�ݣ��������쳣 (δEu =1. 00 ��1. 20��ƽ��ֵ 1. 08); ��Ʒ������������ʯԪ�� (LILE: ��b��Ba��K)������߳�ǿԪ�� (HFSE: Nb��Ta��P��Ti)���߳�ǿԪ�� Zr��Th �ϸ�������ʯ���� C �Ͱ������������Ϊ����þ�����µؿDz������ڲ��ͨ������ԱȺ��ܽ�ǰ���о��ɹ�����Ϊ�����ĸ�ӿ�������γ���������������ɽ�Ӻ�֮�����չ�׶Ρ�

The authors studied the characteristics of U-Pb chronology and geochemistry of trachyte in Zhalax- iageyong of Tuotuohe area��Qinghai�� The results show that the zircon U-Pb age is 32. 39 ��34. 06 Ma��with weigh- ted average age of (33. 09 ±0. 26) Ma��which indicate the trachyte formed in Oligocene�� The rock belongs to high -SiO₂ ��high-K peraluminous-metaluminous shoshonite series�� The ��EE pattern appears rightward sloping with significant fractionation of L��EE and H��EE ��(La/Yb) N =19. 96 ��34. 00��mean =26. 99�ݣ�with slight positive Eu anomalies (δEu =1. 00 ��1. 20��mean =1. 08)�� The samples are characterized by enrichment in large ion litho- phile elements (LILEs: e. g�� ��b��Ba��K)��depletion in high field strength elements (HFSEs: e. g�� Nb��Ta��P�� Ti)��but HFSEs Th and Zr are richer�� The rock has the characteristics of C-type adakite��as a new product of the partial melting of mafic lower crust�� On the basis of the previous research results in combination with regional com- parison��it is considered that Zhalaxiageyong trachyte formed in extensional stage after orogenic thickening in Early Cenozoic��