西藏搭格架热泉型铯矿床年代学研究

Study on chronology for hot spring typed Cs-deposit of Targjia,Tibet.

西藏昂仁县搭格架地热田硅华中的铯达到超大型矿床规模,不同特征的热泉沉积或分布于冈底斯冰期冰碛物的外表或裂隙中,或覆于长马曲上游第5至第1级阶地及现代河床的不同部位。据热泉华野外分布的地貌部位和岩石学特征可将其分为6套。这6套泉华代表性样品的全溶法或等时线法U系测年结果表明:第1套泉华(钙华)形成于晚更新世晚期的末次冰期间冰阶晚期,相当于深海氧同位素第3阶段即MIS 3;第2套泉华(硅华)形成于中更新世晚期的倒数第二次冰期,相当于MIS 8～7;第3套泉华形成于晚更新世早期的末次间冰期,相当于MIS 5;第4套泉华(硅华)形成于晚更新世晚期的末次冰期间冰阶晚期至末次盛冰期早期,相当于MIS 3～2;第5套泉华(硅华)形成于晚更新世末期的末次盛冰期至全新世冰后期,相当于MIS 2～1;第6套泉华即现代硅华。参考前人的ESR年龄测定结果,可将搭格架热泉华的形成分为5个阶段:403～202ka BP,99ka BP左右,39～25ka BP,17～4ka BP与现代。在第3阶段硅华活动早期,有一次钙华活动。

Targjia thermal field, a large scale Cs-deposit, is situated at a source of Raka Zangbo R. , a tributary of Yarlung Zsangpo River. Sinter with different characteristics is distributed on the surface and in the fracture of moraine in Gangdise Glacial Period, or covered on different terraces （ T1 -T5 ） and river bed of Changmaqu River. Based on geomorphic distribution and petrologic features, sinter can be divided into 6 suites. Suite 1 is travertine and the others are geyserite. At present, there is no better method to date for Quaternary sinter, especially geyserite. Before this paper, there are five ESR dates from Targiia geyserite. But we do not know fully about their geomorphic distribution of sampling sites. Two methods of U-series dating, bulk analysis of a whole-sample and totalsample-dissolution isochron techniques （i. e., TSD scheme）, are used in this paper. Chronologic explanation with geomorphic distribution is given as the following： Suite 1 is formed in late stage of interstade in the Last Glaciation of the late Late Pleistocene, i. e. MIS 3. Suite 2, the Penultimate Glacial Period of the late Middle Pleistocene, MIS 8 - 7. Suite 3, the Last Interglacial Period in the early Late Pleistocene, MIS 5. Suite 4, during the late stage of interstade of the Last Glaciation and the early stage of Last Glacial Maximum in the late Late Pleistocene, MIS 3 - 2. Suite 5, the Last Glacial Maximum of the late Late Pleistocene and the Postglacial period of the Holocene, MIS 2 - 1. Suite 6 is existing geyserite. Refering to ESR dates from other scientists, this paper thinks that the Targjia thermal sinter is formed in five stages： 403 -202ka BP,about 99ka BP, 39 -25ka BP, 17 -4ka BP and the present. In the early period of stage 3, there was a activity of travertine.