滇东北峨眉山玄武岩中两阶段自然铜矿化的^(40)Ar/^(39)Ar与U-Th-Pb年龄证据

^(40)Ar/^(39)Arand U-Th-Pb dating for native copper mineralizations of two stages from the Emeishan flood basalts in northeastern Yunnan Province, China

滇东北与峨眉山大火成岩省有关的自然铜矿化分布于鲁甸、茂林和迤车向斜的二叠纪玄武岩与上覆宣威组地层之间。与自然铜矿化有关的蚀变矿物———浊沸石(LD-14-3、YSD-1和YC-1)均给出了一致的40Ar/39Ar坪年龄和等时线年龄(226～228Ma)。这类矿物不存在Ca和Cl的干扰,是合适的40Ar/39Ar定年对象。但它们的Ar封闭温度较低,在100～110℃之间。所得年龄代表了第一次成矿作用的年龄。片沸石的Ar封闭温度低于70℃,无Cl干扰的片沸石LG-2给出了(134.0±1.7)Ma的40Ar/39Ar稳定坪年龄,并与U-Th-Pb等时线定年结果相一致,表明该区在白垩纪早期存在第二次低温热液作用和自然铜矿化。因此无氯沸石40Ar/39Ar定年可广泛应用于低温变质作用和冷却热历史研究。38Ar含量高的阳起石和片沸石样品存在Cl干扰对40Ar/39Ar定年的影响。应用40Ar/38Ar-39Ar/38Ar等时线关系或39Ar/38Ar-36Ar/38Ar相关性进行36Ar修正有可能获得有意义的年龄值;修正后的阳起石年龄为235.7～238.6Ma,片沸石年龄为149.1Ma,可与无氯沸石的年龄相比较。

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, 40Ar/39Ar 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 40Ar/39Ar plateau and isochron ages (226～228 Ma). There are no disturbing from calcium and chlorine for 40Ar/39Ar 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 40Ar/39Ar 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, 40Ar/39Ar dating for zeolites free from chlorine can be widely used for studies of very low temperature metamorphisms and cooling histories. 40Ar/39Ar dating is disturbed by chlorine in the actinolite and heulandite with high 38Ar concentrations. However, significant age results can be obtained using 40Ar/38Ar-39Ar/38Ar isochron plots or 36Ar revision according to a 39Ar/38Ar-36Ar/38Ar linear correlation. The revised 40Ar/39Ar 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.