Ar^(40)/Ar^(39)计时及西藏南部喜马拉雅期几个黑云母年龄的测定

Ar^(40)/Ar^(39) DATING AND SOME ISOTOPIC AGE DETERMINA TIONS ON HIMALAYAN BIOTITES FROM GRANITOID ROCKS IN SOUTHERN XIZANG

Five biotites of Himalayan Period from southern Xizang platean have been irradiated and determined using reactor No. 901 in Qinghua University as fast neutron source(neutron flux 1.17× 10^13n/cm^2·s, fast neutron - 25%, duration of irradiation 18 hours) and MMhb-1 hornblende from USA as standard. Most of the samples were also determined by Ar^36 isotope dilation method, and good agreement has been obtained for the results between the two approaches. This shows that Ar^40/Ar^39 dating is capable to distinguish clearly the order of intrusion among the multiphase intrnsions with respect to the young granites of Himalayan Period. Powder sample of CaF2 and K2SO4 were irradiated and ratios of (Ar^36/Ar^37)Ca. and(Ar^39/Ar^37)Ca and (Ar^40/Ar^39)x have been measured, which are of the same magnitutc of order as reported from other reac-tors in the world. For the ZBH-2506 biotite standard of China, Ar^40/Ar^39 age (130.9--131.9my)agrees with the age of 131±2.6 my as determined by dilution method. Plateau age obtained by step-wise heating (131±0.8my) is similar with the data obtained from total fusion and dilution methods.

Five biotites of Himalayan Period from southern Xizang plateau have been irradiated and determined using reactor No. 901 in Qinghua University as fast neutron source (neutron flux 1.17×1013 n/cm2·s, fast neutron - 25%, duration of irradiation 18 hours) and MMhb-1 hornblende from USA as standard. Most of the samples were also determined by Ar38 isotope dilution method, and good agreement has been obtained for the results between the two approaches. This shows that Ar49/Ar39 dating is capable to distinguish clearly the order of intrusion among the multiphase intrusions with respect to the young granites of Himalayan Period. Powder sample of CaF2 and K2SO4 were irradiated and ratios of (Ar36/Ar37)ca, (Ar39/Ar37)Ca and (Ar40/Ar39)k have been measured, which are of the same magnitute of order as reported from other reactors in the world.For the ZBH-2506 biotite standard of China, Ar40/Ar39 age (130.9-131.9 my) agrees with the age of 131±2.6 my as determined by dilution method. Plateau age obtained by step-wise heating (131±0.8 my) is similar with the data obtained from total fusion and dilution methods.