扬子板块西缘黄草山和下索子花岗岩体锆石U-Pb年代学研究

U-Pb Chronological Study of Zircons from the Huangcaoshan and Xiasuozi Granites in the Western Margin of Yangtze Plate

对扬子板块西缘泸定—石棉地区黄草山花岗岩体和康定地区下索子花岗岩体中自形锆石 ,采用2 0 5Pb - 2 35U混合同位素稀释法测定了锆石U Pb年龄。测试结果表明 ,黄草山岩体中锆石结晶年龄为 786± 36Ma ,下索子岩体中锆石结晶年龄为 80 5± 15Ma ,表明它们都是晋宁期岩浆活动产物。

Idiomorphic grains of zircon from the Huangcaoshan and Xiasuozi granites in the western margin of Yangtze plate were analyzed by using the 206 Pb? 235 U mixed dilution technique. The results show that U?Pb ages of zircons from the Huangcaoshan granite are variable. The apparent?age relationship t 207/206 >t 207/235 >t 206/238 is evident for these zircons, indicating that the difference in apparent ages for different grains is caused by radiogenic Pb loss in different degrees. In the concordance diagram, the lower intercept age of 786±36 Ma represents the crystallization age of the zircon; the upper intercept age of 195±78 Ma may be interpreted as the age of heat event causing the radiogenic Pb loss. Differing from the Huangcaoshan granite, there is no difference in the apparent ages of zircons from the Xiasuozi granite within the analytical error. In the concordance diagram, the data?points are all plotted on the concordance line, indicating that U?Pb isotopic system remains closed after crystallization of zircons. Therefore, the concordance age of 805±15 Ma may represent the crystallization age of zircons. The above?mentioned results show that the Huangcaoshan and Xiasuozi granites were the products of Jinning period magmatism, and the important components of the Jinning orogenic belt around the Yangzte plate.