滇東北峨眉山玄武岩中兩階段自然銅礦化的(上標 40)Ar/(上標 39)Ar與U-Th-Pb年齡證據

The native copper mineralizations related to the Emeishan flood basalts in northeastern Yunnan occur in the Ludian, Maoling and Yiche synclines, and lie between the Permian flood basalts and the overlain strata of Xuanwei Formation. In order to study formation age and evolution history of native copper mineralization, (superscript 40)Ar/(superscript 39)Ar and U-Th-Pb dating for the paragenetic minerals and ores is done. The altered mineral, laumontites associated with native copper mineralizations in the three synclines (LD-14-3, YSD-1 and YC-1) yield consistent (superscript 40)Ar/(superscript 39)Ar plateau and isochron ages (226~228 Ma). There are no disturbing from calcium and chlorine for (superscript 40)Ar/(superscript 39)Ar dating of these minerals. However, their closure temperature of argon is considerable low (100~110℃), thus the data represent a lower age limit of the native copper mineralization of first time. The determined closure temperatures of argon for heulandites are lower than 70℃. The LG-2 heulandite without chlorine disturbing yield a stable (superscript 40)Ar/(superscript 39)Ar plateau age of (134.0±1.7) Ma, and in agreement with the isochron ages of U-Th-Pb isotopic system, which indicates that there existed the second hydrothermal activation and native copper mineralization during Early Cretaceous. Therefore, (superscript 40)Ar/(superscript 39)Ar dating for zeolites free from chlorine can be widely used for studies of very low temperature metamorphisms and cooling histories. (superscript 40)Ar/(superscript 39)Ar dating is disturbed by chlorine in the actinolite and heulandite with high 38Ar concentrations. However, significant age results can be obtained using (superscript 40)Ar/(superscript 38)Ar-(superscript 39)Ar/(superscript 38)Ar isochron plots or (superscript 36)Ar revision according to a (superscript 39)Ar/(superscript 38)Ar-(superscript 36)Ar/(superscript 38)Ar linear correlation. The revised (superscript 40)Ar/(superscript 39)Ar ages for the LD-14 actinolite and heulandite are 235.7~238.6 Ma and 149.1 Ma, respectively, which are comparable with those of the zeolites free from chlorine.