云南怒江丙中洛河段第三级阶地^10Be暴露年龄

^(10)Be DATED BOULDERS FROM THE THIRD TERRACE OF NUJIANG RIVER AT BINGZHONGLUO,YUNNAN PROVINCE,CHINA

宇宙成因核素可用于河流阶地测年,然而保存于阶地面上的漂砾暴露年龄是否能代表其形成年代还缺乏深入研究。应用宇宙成因核素^(10)Be对怒江丙中洛河段第三级阶地上的花岗岩漂砾进行测年研究,结果显示继承性组分可以忽略,而风化侵蚀将对其暴露年龄产生较大影响。基于采集自第三级阶地保存较好的基岩中石英脉样品,应用有效暴露年龄、暴露时间与风化速率间的关系图解出花岗岩漂砾的风化速率为0.3cm/ka,并据此得到第三级阶地的形成年代大约为150～203ka。

Fluvial terraces are common in the Nujiang River valley,while the third level is widest and mostly distributed. There are five level terraces at B ingzhongluo（28 °01 ＇N ,98°37 ＇E） , where located southeastern margin of the Tibetan Plateau,Yunnan, China. The third and the forth level terraces are covered with reddish-brown soil,indicating a long history of the terraces. Many granite boulders are spread out on the surface of the first, second and third level terraces ,whose exposure ages could represent the terrace formation time. Three granite boulders were taken as samples at the posterior margin of the third terrace of the Nujiang River at Bingzhongluo （Fig. la and lc）. The diameters of all boulders are large （〉2m）and partly buried in soil layers. Sample NJ2-1 was collected from one granite boulder（about 2.3m）on floodplaln,which is used to estimate the inheritance. Sample BZIA3 was chiseled about 1.5cm from the surface of a quartz vein,which insets a sandy slate of the third terrace at downstream about 2km. The quartz purification,^10Be extraction,and BeO preparation,were done in the Cosmogenic Nuclide Laboratory at the Institute of Geology and Geophysics, Chinese Academy of Sciences. The l^Be/gBe ratio was measured by AMS facility at the Tandem Laboratory of Uppsala University in Sweden. The ^10Be concentrations（Table 1 ） show that the ^10Be concentration of sample BZIA3 is the highest（ （ 1. 742±0. 043 ）×10^6 atoms/g） of the four samples from the third terrace, the average content of other three samples （N J2-8, N J2-lg and N J2-15 ）from granite boulder is about 1.54×106 atoms/g, and sample N J2-1 from the floodplain is very low, only （1.6±0.7）× 10^4 atoms/g. Weathering processes have a greater impact on the exposure ages of boulder samples according to the landscape,environmental features and ^10Be concentration. Assuming the weathering rate of sample BZL43 from the quartz vein as 0.05cm/ka, the weathering rate of granite boulders is represented graphically as O. 3cm/ka by the relationship among effective exposure age,exposure time and weathering rate （ Fig. 2）. The exposure ages of the three granite boulders on the third terrace were obtained after the correction for weathering and ignoring inheritance. Combined with the corrected exposure age of sample BZIA3, it is considered that the third terrace of Nujiang River at Bingzhongluo was developed during 150 - 203ka.